[flutter_tools] fix performance of tree-shake-icons (#55417)

Fixes the performance issue with tree-shake-icons and filters to ttf mime type. Does not change default or error behavior.

packages/flutter_tools/lib/src/build_system/targets/assets.dart

@@ -60,9 +60,9 @@ Future<Depfile> copyAssets(Environment environment, Directory outputDirectory) a
         file.parent.createSync(recursive: true);
         final DevFSContent content = entry.value;
         if (content is DevFSFileContent && content.file is File) {
-          inputs.add(globals.fs.file(content.file.path));
+          inputs.add(content.file as File);
           if (!await iconTreeShaker.subsetFont(
-            inputPath: content.file.path,
+            input: content.file as File,
             outputPath: file.path,
             relativePath: entry.key,
           )) {

packages/flutter_tools/lib/src/build_system/targets/icon_tree_shaker.dart

@@ -4,6 +4,7 @@
 
 import 'package:meta/meta.dart';
 import 'package:process/process.dart';
+import 'package:mime/mime.dart' as mime;
 
 import '../../artifacts.dart';
 import '../../base/common.dart';
@@ -66,6 +67,12 @@ class IconTreeShaker {
     }
   }
 
+  /// The MIME type for ttf fonts.
+  static const Set<String> kTtfMimeTypes = <String>{
+    'font/ttf', // based on internet search
+    'application/x-font-ttf', // based on running locally.
+  };
+
   /// The [Source] inputs that targets using this should depend on.
   ///
   /// See [Target.inputs].
@@ -77,6 +84,7 @@ class IconTreeShaker {
 
   final Environment _environment;
   final String _fontManifest;
+  Future<void> _iconDataProcessing;
   Map<String, _IconTreeShakerData> _iconData;
 
   final ProcessManager _processManager;
@@ -89,10 +97,10 @@ class IconTreeShaker {
                    && _environment.defines[kIconTreeShakerFlag] == 'true'
                    && _environment.defines[kBuildMode] != 'debug';
 
-  /// Fills the [_iconData] map.
-  Future<Map<String, _IconTreeShakerData>> _getIconData(Environment environment) async {
+  // Fills the [_iconData] map.
+  Future<void> _getIconData(Environment environment) async {
     if (!enabled) {
-      return null;
+      return;
     }
 
     final File appDill = environment.buildDir.childFile('app.dill');
@@ -119,12 +127,14 @@ class IconTreeShaker {
     );
 
     if (fonts.length != iconData.length) {
-      throwToolExit('Expected to find fonts for ${iconData.keys}, but found '
-                    '${fonts.keys}. This usually means you are refering to '
-                    'font families in an IconData class but not including them '
-                    'in the assets section of your pubspec.yaml, are missing '
-                    'the package that would include them, or are missing '
-                    '"uses-material-design: true".');
+      throwToolExit(
+        'Expected to find fonts for ${iconData.keys}, but found '
+        '${fonts.keys}. This usually means you are refering to '
+        'font families in an IconData class but not including them '
+        'in the assets section of your pubspec.yaml, are missing '
+        'the package that would include them, or are missing '
+        '"uses-material-design: true".',
+      );
     }
 
     final Map<String, _IconTreeShakerData> result = <String, _IconTreeShakerData>{};
@@ -135,13 +145,11 @@ class IconTreeShaker {
         codePoints: iconData[entry.key],
       );
     }
-    return result;
+    _iconData = result;
   }
 
-  /// Calls font-subset, which transforms the `inputPath` font file to a
-  /// subsetted version at `outputPath`.
-  ///
-  /// The `relativePath` parameter
+  /// Calls font-subset, which transforms the [input] font file to a
+  /// subsetted version at [outputPath].
   ///
   /// All parameters are required.
   ///
@@ -150,15 +158,24 @@ class IconTreeShaker {
   /// If the font-subset subprocess fails, it will [throwToolExit].
   /// Otherwise, it will return true.
   Future<bool> subsetFont({
-    @required String inputPath,
+    @required File input,
     @required String outputPath,
     @required String relativePath,
   }) async {
     if (!enabled) {
       return false;
     }
-
-    _iconData ??= await _getIconData(_environment);
+    if (input.lengthSync() < 12) {
+      return false;
+    }
+    final String mimeType = mime.lookupMimeType(
+      input.path,
+      headerBytes: await input.openRead(0, 12).first,
+    );
+    if (!kTtfMimeTypes.contains(mimeType)) {
+      return false;
+    }
+    await (_iconDataProcessing ??= _getIconData(_environment));
     assert(_iconData != null);
 
     final _IconTreeShakerData iconTreeShakerData = _iconData[relativePath];
@@ -176,7 +193,7 @@ class IconTreeShaker {
     final List<String> cmd = <String>[
       fontSubset.path,
       outputPath,
-      inputPath,
+      input.path,
     ];
     final String codePoints = iconTreeShakerData.codePoints.join(' ');
     _logger.printTrace('Running font-subset: ${cmd.join(' ')}, '
@@ -186,9 +203,7 @@ class IconTreeShaker {
       fontSubsetProcess.stdin.writeln(codePoints);
       await fontSubsetProcess.stdin.flush();
       await fontSubsetProcess.stdin.close();
-    } on Exception catch (_) {
-      // handled by checking the exit code.
-    } on OSError catch (_) {  // ignore: dead_code_on_catch_subtype
+    } on Exception {
       // handled by checking the exit code.
     }
 

packages/flutter_tools/lib/src/commands/build_bundle.dart

@@ -136,7 +136,7 @@ class BuildBundleCommand extends BuildSubCommand {
       extraGenSnapshotOptions: buildInfo.extraGenSnapshotOptions,
       fileSystemScheme: stringArg('filesystem-scheme'),
       fileSystemRoots: stringsArg('filesystem-root'),
-      treeShakeIcons: boolArg('tree-shake-icons'),
+      treeShakeIcons: buildInfo.treeShakeIcons,
     );
     return FlutterCommandResult.success();
   }

packages/flutter_tools/lib/src/commands/build_web.dart

@@ -19,7 +19,7 @@ class BuildWebCommand extends BuildSubCommand {
   BuildWebCommand({
     @required bool verboseHelp,
   }) {
-    addTreeShakeIconsFlag();
+    addTreeShakeIconsFlag(enabledByDefault: false);
     usesTargetOption();
     usesPubOption();
     addBuildModeFlags(excludeDebug: true);

packages/flutter_tools/lib/src/runner/flutter_command.dart

@@ -425,10 +425,13 @@ abstract class FlutterCommand extends Command<void> {
     );
   }
 
-  void addTreeShakeIconsFlag() {
+  void addTreeShakeIconsFlag({
+    bool enabledByDefault
+  }) {
     argParser.addFlag('tree-shake-icons',
       negatable: true,
-      defaultsTo: kIconTreeShakerEnabledDefault,
+      defaultsTo: enabledByDefault
+        ?? kIconTreeShakerEnabledDefault,
       help: 'Tree shake icon fonts so that only glyphs used by the application remain.',
     );
   }
@@ -579,8 +582,12 @@ abstract class FlutterCommand extends Command<void> {
         'combination with "--${FlutterOptions.kSplitDebugInfoOption}"',
       );
     }
+    final BuildMode buildMode = getBuildMode();
+    final bool treeShakeIcons = argParser.options.containsKey('tree-shake-icons')
+      && buildMode.isPrecompiled
+      && boolArg('tree-shake-icons');
 
-    return BuildInfo(getBuildMode(),
+    return BuildInfo(buildMode,
       argParser.options.containsKey('flavor')
         ? stringArg('flavor')
         : null,
@@ -601,9 +608,7 @@ abstract class FlutterCommand extends Command<void> {
       buildName: argParser.options.containsKey('build-name')
           ? stringArg('build-name')
           : null,
-      treeShakeIcons: argParser.options.containsKey('tree-shake-icons')
-          ? boolArg('tree-shake-icons')
-          : kIconTreeShakerEnabledDefault,
+      treeShakeIcons: treeShakeIcons,
       splitDebugInfoPath: splitDebugInfoPath,
       dartObfuscation: dartObfuscation,
       dartDefines: argParser.options.containsKey(FlutterOptions.kDartDefinesOption)