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

pkg/compiler/lib/src/resolution/send_structure.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.resolution.send_structure;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
 import '../elements/elements.dart';
@@ skipping 2185 matching lines @@
 /// `const C()`.
 class ConstInvokeStructure<R, A> extends NewStructure<R, A> {
   final ConstantInvokeKind constantInvokeKind;
   final ConstantExpression constant;
 
   ConstInvokeStructure(this.constantInvokeKind, this.constant);
 
   @override
   NewStructureKind get kind => NewStructureKind.CONST_INVOKE;
 
+  // ignore: MISSING_RETURN
   R dispatch(SemanticSendVisitor<R, A> visitor, NewExpression node, A arg) {
     switch (constantInvokeKind) {
       case ConstantInvokeKind.CONSTRUCTED:
         return visitor.visitConstConstructorInvoke(node, constant, arg);
       case ConstantInvokeKind.BOOL_FROM_ENVIRONMENT:
         return visitor.visitBoolFromEnvironmentConstructorInvoke(
             node, constant, arg);
       case ConstantInvokeKind.INT_FROM_ENVIRONMENT:
         return visitor.visitIntFromEnvironmentConstructorInvoke(
             node, constant, arg);
@@ skipping 178 matching lines @@
 
   R dispatch(SemanticDeclarationVisitor<R, A> visitor,
       VariableDefinitions definitions, A arg);
 }
 
 class NonConstantVariableStructure<R, A> extends VariableStructure<R, A> {
   NonConstantVariableStructure(
       VariableKind kind, Node node, VariableElement variable)
       : super(kind, node, variable);
 
+  // ignore: MISSING_RETURN
   R dispatch(SemanticDeclarationVisitor<R, A> visitor,
       VariableDefinitions definitions, A arg) {
     switch (kind) {
       case VariableKind.TOP_LEVEL_FIELD:
         return visitor.visitTopLevelFieldDeclaration(
             definitions, node, variable, variable.initializer, arg);
       case VariableKind.STATIC_FIELD:
         return visitor.visitStaticFieldDeclaration(
             definitions, node, variable, variable.initializer, arg);
       case VariableKind.INSTANCE_FIELD:
@@ skipping 98 matching lines @@
   ThisConstructorInvokeStructure(
       this.node, this.constructor, this.callStructure);
 
   R dispatch(SemanticDeclarationVisitor<R, A> visitor, A arg) {
     return visitor.visitThisConstructorInvoke(
         node, constructor, node.argumentsNode, callStructure, arg);
   }
 
   bool get isConstructorInvoke => true;
 }