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

pkg/compiler/lib/src/constants/expressions.dart

 // Copyright (c) 2014, 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.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../common_elements.dart';
 import '../elements/types.dart';
@@ skipping 923 matching lines @@
             .lookupBinary(operator)
             .fold(leftValue, rightValue);
     }
   }
 
   ConstantExpression apply(NormalizedArguments arguments) {
     return new BinaryConstantExpression(
         left.apply(arguments), operator, right.apply(arguments));
   }
 
+  // ignore: MISSING_RETURN
   InterfaceType getKnownType(CommonElements commonElements) {
     DartType knownLeftType = left.getKnownType(commonElements);
     DartType knownRightType = right.getKnownType(commonElements);
     switch (operator.kind) {
       case BinaryOperatorKind.EQ:
       case BinaryOperatorKind.NOT_EQ:
       case BinaryOperatorKind.LOGICAL_AND:
       case BinaryOperatorKind.LOGICAL_OR:
       case BinaryOperatorKind.GT:
       case BinaryOperatorKind.LT:
@@ skipping 987 matching lines @@
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
 }