Make some ErrorCode-s compile-time errors and some just type warnings

compiler/java/com/google/dart/compiler/DartCompilerMainContext.java

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 package com.google.dart.compiler;
 
 import com.google.dart.compiler.ast.DartUnit;
 import com.google.dart.compiler.ast.LibraryUnit;
 import com.google.dart.compiler.metrics.CompilerMetrics;
 import com.google.dart.compiler.parser.DartParser;
-import com.google.dart.compiler.resolver.ResolverErrorCode;
 import com.google.dart.compiler.resolver.TypeErrorCode;
 
 import java.io.IOException;
 import java.io.Reader;
 import java.io.Writer;
 import java.net.URI;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
 /**
@@ skipping 155 matching lines @@
   @Override
   public LibrarySource getSystemLibraryFor(String importSpec) {
     return compilerConfiguration.getSystemLibraryFor(importSpec);
   }
 
   @Override
   public void unitCompiled(DartUnit unit) {
     listener.unitCompiled(unit);
   }
 }