Explicitly report CFE output(when using --preview-dart-2) (#13936)

* Explicitly report CFE output(when using --preview-dart-2)

* Fix test so it looks at errorText

* Fix expected text

packages/flutter_tools/lib/src/compile.dart

@@ -45,7 +45,7 @@ class _StdoutHandler {
         ? string.substring(boundaryKey.length + 1)
         : null);
     else
-      printTrace('compile debug message: $string');
+      printError('compiler message: $string');
   }
 
   // This is needed to get ready to process next compilation result output,
@@ -98,14 +98,14 @@ Future<String> compile(
   final Process server = await processManager
       .start(command)
       .catchError((dynamic error, StackTrace stack) {
-    printTrace('Failed to start frontend server $error, $stack');
+    printError('Failed to start frontend server $error, $stack');
   });
 
   final _StdoutHandler stdoutHandler = new _StdoutHandler();
 
   server.stderr
     .transform(UTF8.decoder)
-    .listen((String s) { printTrace('compile debug message: $s'); });
+    .listen((String s) { printError('compiler message: $s'); });
   server.stdout
     .transform(UTF8.decoder)
     .transform(const LineSplitter())
@@ -171,7 +171,7 @@ class ResidentCompiler {
     _server.stderr
       .transform(UTF8.decoder)
       .transform(const LineSplitter())
-      .listen((String s) { printTrace('compile debug message: $s'); });
+      .listen((String s) { printError('compiler message: $s'); });
 
     _server.stdin.writeln('compile $scriptFilename');
 

packages/flutter_tools/test/compile_test.dart

@@ -49,7 +49,7 @@ void main() {
         mainPath: '/path/to/main.dart'
       );
       expect(mockFrontendServerStdIn.getAndClear(), isEmpty);
-      expect(logger.traceText, equals('compile debug message: line1\ncompile debug message: line2\n'));
+      expect(logger.errorText, equals('compiler message: line1\ncompiler message: line2\n'));
       expect(output, equals('/path/to/main.dart.dill'));
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
@@ -69,7 +69,7 @@ void main() {
         mainPath: '/path/to/main.dart'
       );
       expect(mockFrontendServerStdIn.getAndClear(), isEmpty);
-      expect(logger.traceText, equals('compile debug message: line1\ncompile debug message: line2\n'));
+      expect(logger.errorText, equals('compiler message: line1\ncompiler message: line2\n'));
       expect(output, equals(null));
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
@@ -119,7 +119,7 @@ void main() {
       );
       expect(mockFrontendServerStdIn.getAndClear(), 'compile /path/to/main.dart\n');
       verifyNoMoreInteractions(mockFrontendServerStdIn);
-      expect(logger.traceText, equals('compile debug message: line1\ncompile debug message: line2\n'));
+      expect(logger.errorText, equals('compiler message: line1\ncompiler message: line2\n'));
       expect(output, equals('/path/to/main.dart.dill'));
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
@@ -140,9 +140,9 @@ void main() {
 
       verifyNoMoreInteractions(mockFrontendServerStdIn);
       expect(mockFrontendServerStdIn.getAndClear(), isEmpty);
-      expect(logger.traceText, equals(
-        'compile debug message: line0\ncompile debug message: line1\n'
-        'compile debug message: line1\ncompile debug message: line2\n'
+      expect(logger.errorText, equals(
+        'compiler message: line0\ncompiler message: line1\n'
+        'compiler message: line1\ncompiler message: line2\n'
       ));
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,
@@ -167,10 +167,10 @@ void main() {
 
       verifyNoMoreInteractions(mockFrontendServerStdIn);
       expect(mockFrontendServerStdIn.getAndClear(), isEmpty);
-      expect(logger.traceText, equals(
-        'compile debug message: line0\ncompile debug message: line1\n'
-        'compile debug message: line1\ncompile debug message: line2\n'
-        'compile debug message: line2\ncompile debug message: line3\n'
+      expect(logger.errorText, equals(
+        'compiler message: line0\ncompiler message: line1\n'
+        'compiler message: line1\ncompiler message: line2\n'
+        'compiler message: line2\ncompiler message: line3\n'
       ));
     }, overrides: <Type, Generator>{
       ProcessManager: () => mockProcessManager,