Use DiagnosticMessage in MockCompiler.

tests/compiler/dart2js/type_test_helper.dart

Index: tests/compiler/dart2js/type_test_helper.dart
diff --git a/tests/compiler/dart2js/type_test_helper.dart b/tests/compiler/dart2js/type_test_helper.dart
index e4c3ce3ce25cae667f928233021af07c8ef02f0b..df060084df170879535040f8004146baf40ddeca 100644
--- a/tests/compiler/dart2js/type_test_helper.dart
+++ b/tests/compiler/dart2js/type_test_helper.dart
@@ -37,8 +37,6 @@ class TypeEnvironment {
                       bool stopAfterTypeInference: false,
                       String mainSource}) {
     Uri uri;
-    Function getErrors;
-    Function getWarnings;
     Compiler compiler;
     bool stopAfterTypeInference = mainSource != null;
     if (mainSource == null) {
@@ -48,6 +46,7 @@ class TypeEnvironment {
     } else {
       source = '$mainSource\n$source';
     }
+    memory.DiagnosticCollector collector;
     if (useMockCompiler) {
       uri = new Uri(scheme: 'source');
       mock.MockCompiler mockCompiler = mock.compilerFor(
@@ -56,29 +55,26 @@ class TypeEnvironment {
           analyzeAll: !stopAfterTypeInference,
           analyzeOnly: !stopAfterTypeInference);
       mockCompiler.diagnosticHandler = mock.createHandler(mockCompiler, source);
-      getErrors = () => mockCompiler.errors;
-      getWarnings = () => mockCompiler.warnings;
+      collector = mockCompiler.diagnosticCollector;
       compiler = mockCompiler;
     } else {
-      memory.DiagnosticCollector collector = new memory.DiagnosticCollector();
+      collector = new memory.DiagnosticCollector();
       uri = Uri.parse('memory:main.dart');
       compiler = memory.compilerFor(
           memorySourceFiles: {'main.dart': source},
           diagnosticHandler: collector,
           options: stopAfterTypeInference
               ? [] : [Flags.analyzeAll, Flags.analyzeOnly]);
-      getErrors = () => collector.errors;
-      getWarnings = () => collector.warnings;
     }
     compiler.stopAfterTypeInference = stopAfterTypeInference;
     return compiler.run(uri).then((_) {
       if (expectNoErrors || expectNoWarningsOrErrors) {
-        var errors = getErrors();
+        var errors = collector.errors;
         Expect.isTrue(errors.isEmpty,
             'Unexpected errors: ${errors}');
       }
       if (expectNoWarningsOrErrors) {
-        var warnings = getWarnings();
+        var warnings = collector.warnings;
         Expect.isTrue(warnings.isEmpty,
             'Unexpected warnings: ${warnings}');
       }