Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/tree/nodes.dart

 // Copyright (c) 2012, 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.
 
 import 'dart:collection' show IterableMixin;
 
 import '../common.dart';
 import '../elements/elements.dart' show MetadataAnnotation;
 import 'package:front_end/src/fasta/scanner/precedence.dart' as Precedence
     show FUNCTION_INFO;
@@ skipping 221 matching lines @@
   R visitMixinApplication(MixinApplication node, A arg) => visitNode(node, arg);
   R visitModifiers(Modifiers node, A arg) => visitNode(node, arg);
   R visitNamedArgument(NamedArgument node, A arg) => visitExpression(node, arg);
   R visitNamedMixinApplication(NamedMixinApplication node, A arg) {
     return visitMixinApplication(node, arg);
   }
 
   R visitNewExpression(NewExpression node, A arg) => visitExpression(node, arg);
   R visitNodeList(NodeList node, A arg) => visitNode(node, arg);
   R visitNominalTypeAnnotation(NominalTypeAnnotation node, A arg) {
-    visitTypeAnnotation(node, arg);
+    return visitTypeAnnotation(node, arg);
   }
 
   R visitOperator(Operator node, A arg) => visitIdentifier(node, arg);
   R visitParenthesizedExpression(ParenthesizedExpression node, A arg) {
     return visitExpression(node, arg);
   }
 
   R visitPart(Part node, A arg) => visitLibraryTag(node, arg);
   R visitPartOf(PartOf node, A arg) => visitNode(node, arg);
   R visitPostfix(Postfix node, A arg) => visitNodeList(node, arg);
@@ skipping 958 matching lines @@
   LiteralInt asLiteralInt() => this;
 
   int get value {
     try {
       Token valueToken = token;
       if (identical(valueToken.kind, Tokens.PLUS_TOKEN)) {
         valueToken = valueToken.next;
       }
       return int.parse(valueToken.lexeme);
     } on FormatException catch (ex) {
-      (this.handler)(token, ex);
+      throw handler(token, ex);
     }
   }
 
   accept(Visitor visitor) => visitor.visitLiteralInt(this);
 
   accept1(Visitor1 visitor, arg) => visitor.visitLiteralInt(this, arg);
 }
 
 class LiteralDouble extends Literal<double> {
   LiteralDouble(Token token, DecodeErrorHandler handler)
       : super(token, handler);
 
   LiteralDouble asLiteralDouble() => this;
 
   double get value {
     try {
       Token valueToken = token;
       if (identical(valueToken.kind, Tokens.PLUS_TOKEN)) {
         valueToken = valueToken.next;
       }
       return double.parse(valueToken.lexeme);
     } on FormatException catch (ex) {
-      (this.handler)(token, ex);
+      throw handler(token, ex);
     }
   }
 
   accept(Visitor visitor) => visitor.visitLiteralDouble(this);
 
   accept1(Visitor1 visitor, arg) => visitor.visitLiteralDouble(this, arg);
 }
 
 class LiteralBool extends Literal<bool> {
   LiteralBool(Token token, DecodeErrorHandler handler) : super(token, handler);
@@ skipping 2023 matching lines @@
  */
 Object getTreeElement(TreeElementMixin node) => node._element;
 
 /**
  * Do not call this method directly.  Instead, use an instance of
  * TreeElements.
  */
 void setTreeElement(TreeElementMixin node, Object value) {
   node._element = value;
 }