Extract DiagnosticReporter implementation from Compiler.

pkg/compiler/lib/src/js_backend/type_variable_handler.dart

Index: pkg/compiler/lib/src/js_backend/type_variable_handler.dart
diff --git a/pkg/compiler/lib/src/js_backend/type_variable_handler.dart b/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
index bfc973f318aafc2f077a099b080eb9cc145b56ed..ec6eecf46cf3a3306e96760b6e615a9fb6a57b00 100644
--- a/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
+++ b/pkg/compiler/lib/src/js_backend/type_variable_handler.dart
@@ -36,6 +36,7 @@ class TypeVariableHandler {
   CodeEmitterTask get _task => _backend.emitter;
   MetadataCollector get _metadataCollector => _task.metadataCollector;
   JavaScriptBackend get _backend => _compiler.backend;
+  DiagnosticReporter get reporter => _compiler.reporter;
 
   void registerClassWithTypeVariables(ClassElement cls, Enqueuer enqueuer,
                                       Registry registry) {
@@ -47,7 +48,7 @@ class TypeVariableHandler {
         _typeVariableClass.ensureResolved(_compiler.resolution);
         Link constructors = _typeVariableClass.constructors;
         if (constructors.isEmpty && constructors.tail.isEmpty) {
-          _compiler.internalError(_typeVariableClass,
+          reporter.internalError(_typeVariableClass,
               "Class '$_typeVariableClass' should only have one constructor");
         }
         _typeVariableConstructor = _typeVariableClass.constructors.head;