Fixed regex to show missing assets file error (#131160)

Added Regex to match error message from verbos build as suggested by @stuartmorgan [here](https://github.com/flutter/flutter/pull/98137#discussion_r810559589).
Modified Windows Build Test

Fixes #97065

packages/flutter_tools/lib/src/windows/build_windows.dart

@@ -190,7 +190,16 @@ Future<void> _runBuild(
 
   // MSBuild sends all output to stdout, including build errors. This surfaces
   // known error patterns.
-  final RegExp errorMatcher = RegExp(r':\s*(?:warning|(?:fatal )?error).*?:');
+  final RegExp errorMatcher = RegExp(
+    <String>[
+      // Known error messages
+      r'(:\s*(?:warning|(?:fatal )?error).*?:)',
+      r'Error detected in pubspec\.yaml:',
+
+      // Known secondary error lines for pubspec.yaml
+      r'No file or variants found for asset:',
+    ].join('|'),
+  );
 
   int result;
   try {

packages/flutter_tools/test/commands.shard/hermetic/build_windows_test.dart

@@ -962,6 +962,41 @@ if %errorlevel% neq 0 goto :VCEnd</Command>
     ProcessManager: () => FakeProcessManager.any(),
     FeatureFlags: () => TestFeatureFlags(isWindowsEnabled: true),
   });
+
+  // Tests the case where stdout contains the error about pubspec.yaml
+  // And tests the case where stdout contains the error about missing assets
+  testUsingContext('Windows build extracts errors related to pubspec.yaml from stdout', () async {
+    final FakeVisualStudio fakeVisualStudio = FakeVisualStudio();
+    final BuildWindowsCommand command = BuildWindowsCommand(logger: BufferLogger.test())
+      ..visualStudioOverride = fakeVisualStudio;
+    setUpMockProjectFilesForBuild();
+
+    const String stdout = r'''
+Error detected in pubspec.yaml:
+No file or variants found for asset: images/a_dot_burr.jpeg.
+''';
+
+    processManager = FakeProcessManager.list(<FakeCommand>[
+      cmakeGenerationCommand(),
+      buildCommand('Release',
+        stdout: stdout,
+      ),
+    ]);
+
+    await createTestCommandRunner(command).run(
+      const <String>['windows', '--no-pub']
+    );
+    // Just the warnings and errors should be surfaced.
+    expect(testLogger.errorText, r'''
+Error detected in pubspec.yaml:
+No file or variants found for asset: images/a_dot_burr.jpeg.
+''');
+  }, overrides: <Type, Generator>{
+    FileSystem: () => fileSystem,
+    ProcessManager: () => processManager,
+    Platform: () => windowsPlatform,
+    FeatureFlags: () => TestFeatureFlags(isWindowsEnabled: true),
+  });
 }
 
 class FakeVisualStudio extends Fake implements VisualStudio {