Improves SDK Search Error Messages (#23884)

* Improves error message and adds test.

packages/flutter_tools/lib/src/android/android_sdk.dart

@@ -317,14 +317,36 @@ class AndroidSdk {
 
   String get avdManagerPath => getAvdManagerPath();
 
+  Directory get _platformsDir => fs.directory(fs.path.join(directory, 'platforms'));
+
+  Iterable<Directory> get _platforms {
+    Iterable<Directory> platforms = <Directory>[];
+    if (_platformsDir.existsSync()) {
+      platforms = _platformsDir
+        .listSync()
+        .whereType<Directory>();
+    }
+    return platforms;
+  }
+
   /// Validate the Android SDK. This returns an empty list if there are no
   /// issues; otherwise, it returns a list of issues found.
   List<String> validateSdkWellFormed() {
     if (!processManager.canRun(adbPath))
       return <String>['Android SDK file not found: $adbPath.'];
 
-    if (sdkVersions.isEmpty || latestVersion == null)
-      return <String>['Android SDK is missing command line tools; download from https://goo.gl/XxQghQ'];
+    if (sdkVersions.isEmpty || latestVersion == null) {
+      final StringBuffer msg = StringBuffer('No valid Android SDK platforms found in ${_platformsDir.path}.');
+      if (_platforms.isEmpty) {
+        msg.write(' Directory was empty.');
+      } else {
+        msg.write(' Candidates were:\n');
+        msg.write(_platforms
+          .map((Directory dir) => '  - ${dir.basename}')
+          .join('\n'));
+      }
+      return <String>[msg.toString()];
+    }
 
     return latestVersion.validateSdkWellFormed();
   }
@@ -355,15 +377,6 @@ class AndroidSdk {
   }
 
   void _init() {
-    Iterable<Directory> platforms = <Directory>[]; // android-22, ...
-
-    final Directory platformsDir = fs.directory(fs.path.join(directory, 'platforms'));
-    if (platformsDir.existsSync()) {
-      platforms = platformsDir
-        .listSync()
-        .whereType<Directory>();
-    }
-
     List<Version> buildTools = <Version>[]; // 19.1.0, 22.0.1, ...
 
     final Directory buildToolsDir = fs.directory(fs.path.join(directory, 'build-tools'));
@@ -382,7 +395,7 @@ class AndroidSdk {
     }
 
     // Match up platforms with the best corresponding build-tools.
-    _sdkVersions = platforms.map<AndroidSdkVersion>((Directory platformDir) {
+    _sdkVersions = _platforms.map<AndroidSdkVersion>((Directory platformDir) {
       final String platformName = platformDir.basename;
       int platformVersion;
 

packages/flutter_tools/test/android/android_sdk_test.dart

@@ -83,6 +83,23 @@ void main() {
       ProcessManager: () => processManager,
     });
 
+    testUsingContext('returns validate sdk is well formed', () {
+      sdkDir = MockBrokenAndroidSdk.createSdkDirectory();
+      Config.instance.setValue('android-sdk', sdkDir.path);
+
+      final AndroidSdk sdk = AndroidSdk.locateAndroidSdk();
+      when(processManager.canRun(sdk.adbPath)).thenReturn(true);
+
+      final List<String> validationIssues = sdk.validateSdkWellFormed();
+      expect(validationIssues.first, 'No valid Android SDK platforms found in'
+        ' /.tmp_rand0/flutter_mock_android_sdk.rand0/platforms. Candidates were:\n'
+        '  - android-22\n'
+        '  - android-23');
+    }, overrides: <Type, Generator>{
+      FileSystem: () => fs,
+      ProcessManager: () => processManager,
+    });
+
     testUsingContext('does not throw on sdkmanager version check failure', () {
       sdkDir = MockAndroidSdk.createSdkDirectory();
       Config.instance.setValue('android-sdk', sdkDir.path);
@@ -164,3 +181,34 @@ void main() {
     });
   });
 }
+
+/// A broken SDK installation.
+class MockBrokenAndroidSdk extends Mock implements AndroidSdk {
+  static Directory createSdkDirectory({
+    bool withAndroidN = false,
+    String withNdkDir,
+    bool withNdkSysroot = false,
+    bool withSdkManager = true,
+  }) {
+    final Directory dir = fs.systemTempDirectory.createTempSync('flutter_mock_android_sdk.');
+
+    _createSdkFile(dir, 'platform-tools/adb');
+
+    _createSdkFile(dir, 'build-tools/sda/aapt');
+    _createSdkFile(dir, 'build-tools/af/aapt');
+    _createSdkFile(dir, 'build-tools/ljkasd/aapt');
+
+    _createSdkFile(dir, 'platforms/android-22/android.jar');
+    _createSdkFile(dir, 'platforms/android-23/android.jar');
+
+    return dir;
+  }
+
+  static void _createSdkFile(Directory dir, String filePath, { String contents }) {
+    final File file = dir.childFile(filePath);
+    file.createSync(recursive: true);
+    if (contents != null) {
+      file.writeAsStringSync(contents, flush: true);
+    }
+  }
+}