More flexibility for defining the locations of assets packaged in an FLX (#4462)

* Add a map of relative to absolute paths for assets located outside the app's
  source directory
* If a "packages" directory exists, obtain assets from there instead of
  using the packages/ prefix to indicate package map lookup

packages/flutter_tools/lib/flutter_tools.dart

@@ -13,6 +13,7 @@ Future<int> assembleFlx({
   Map<String, dynamic> manifestDescriptor: const <String, dynamic>{},
   File snapshotFile: null,
   String assetBasePath: flx.defaultAssetBasePath,
+  Map<String, String> assetPathOverrides: const <String, String>{},
   String outputPath: flx.defaultFlxOutputPath,
   String privateKeyPath: flx.defaultPrivateKeyPath
 }) async {
@@ -20,6 +21,7 @@ Future<int> assembleFlx({
     manifestDescriptor: manifestDescriptor,
     snapshotFile: snapshotFile,
     assetBasePath: assetBasePath,
+    assetPathOverrides: assetPathOverrides,
     outputPath: outputPath,
     privateKeyPath: privateKeyPath
   );

packages/flutter_tools/lib/src/flx.dart

@@ -129,7 +129,8 @@ List<_Asset> _getMaterialAssets(String fontSet) {
 Map<_Asset, List<_Asset>> _parseAssets(
   PackageMap packageMap,
   Map<String, dynamic> manifestDescriptor,
-  String assetBase, {
+  String assetBase,
+  Map<String, String> assetPathOverrides, {
   List<String> excludeDirs: const <String>[]
 }) {
   Map<_Asset, List<_Asset>> result = <_Asset, List<_Asset>>{};
@@ -142,7 +143,7 @@ Map<_Asset, List<_Asset>> _parseAssets(
 
   if (manifestDescriptor.containsKey('assets')) {
     for (String asset in manifestDescriptor['assets']) {
-      _Asset baseAsset = _resolveAsset(packageMap, assetBase, asset);
+      _Asset baseAsset = _resolveAsset(packageMap, assetBase, assetPathOverrides, asset);
 
       if (!baseAsset.assetFileExists) {
         printError('Error: unable to locate asset entry in flutter.yaml: "$asset".');
@@ -153,7 +154,7 @@ Map<_Asset, List<_Asset>> _parseAssets(
       result[baseAsset] = variants;
 
       // Find asset variants
-      String assetPath = path.join(baseAsset.base, baseAsset.relativePath);
+      String assetPath = baseAsset.assetFile.path;
       String assetFilename = path.basename(assetPath);
       Directory assetDir = new Directory(path.dirname(assetPath));
 
@@ -188,7 +189,7 @@ Map<_Asset, List<_Asset>> _parseAssets(
         String asset = font['asset'];
         if (asset == null) continue;
 
-        _Asset baseAsset = _resolveAsset(packageMap, assetBase, asset);
+        _Asset baseAsset = _resolveAsset(packageMap, assetBase, assetPathOverrides, asset);
         if (!baseAsset.assetFileExists) {
           printError('Error: unable to locate asset entry in flutter.yaml: "$asset".');
           return null;
@@ -202,8 +203,22 @@ Map<_Asset, List<_Asset>> _parseAssets(
   return result;
 }
 
-_Asset _resolveAsset(PackageMap packageMap, String assetBase, String asset) {
-  if (asset.startsWith('packages/')) {
+_Asset _resolveAsset(
+  PackageMap packageMap,
+  String assetBase,
+  Map<String, String> assetPathOverrides,
+  String asset
+) {
+  String overridePath = assetPathOverrides[asset];
+  if (overridePath != null) {
+    return new _Asset(
+      base: path.dirname(overridePath),
+      source: path.basename(overridePath),
+      relativePath: asset
+    );
+  }
+
+  if (asset.startsWith('packages/') && !FileSystemEntity.isFileSync(path.join(assetBase, asset))) {
     // Convert packages/flutter_gallery_assets/clouds-0.png to clouds-0.png.
     String packageKey = asset.substring(9);
     String relativeAsset = asset;
@@ -374,6 +389,7 @@ Future<int> assemble({
   Map<String, dynamic> manifestDescriptor: const <String, dynamic>{},
   File snapshotFile,
   String assetBasePath: defaultAssetBasePath,
+  Map<String, String> assetPathOverrides: const <String, String>{},
   String outputPath: defaultFlxOutputPath,
   String privateKeyPath: defaultPrivateKeyPath,
   String workingDirPath: defaultWorkingDirPath,
@@ -385,6 +401,7 @@ Future<int> assemble({
     new PackageMap(path.join(assetBasePath, '.packages')),
     manifestDescriptor,
     assetBasePath,
+    assetPathOverrides,
     excludeDirs: <String>[workingDirPath, path.join(assetBasePath, 'build')]
   );