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

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

Index: pkg/compiler/lib/src/resolution/resolution.dart
diff --git a/pkg/compiler/lib/src/resolution/resolution.dart b/pkg/compiler/lib/src/resolution/resolution.dart
index a28490a9b274c8b05dd7be25157fb87362977d57..64c5fc62c2237f154ea2f3388445d834bc92d2c9 100644
--- a/pkg/compiler/lib/src/resolution/resolution.dart
+++ b/pkg/compiler/lib/src/resolution/resolution.dart
@@ -803,17 +803,19 @@ class ResolverTask extends CompilerTask {
     // well?
     List<Element> constConstructors = <Element>[];
     List<Element> nonFinalInstanceFields = <Element>[];
-    cls.forEachMember((holder, member) {
+    cls.forEachMember((holder, dynamic member) {
       reporter.withCurrentElement(member, () {
         // Perform various checks as side effect of "computing" the type.
         member.computeType(resolution);
 
         // Check modifiers.
+        // ignore: UNDEFINED_GETTER
         if (member.isFunction && member.modifiers.isFinal) {
           reporter.reportErrorMessage(
               member, MessageKind.ILLEGAL_FINAL_METHOD_MODIFIER);
         }
         if (member.isConstructor) {
+          // ignore: UNDEFINED_GETTER
           final mismatchedFlagsBits = member.modifiers.flags &
               (Modifiers.FLAG_STATIC | Modifiers.FLAG_ABSTRACT);
           if (mismatchedFlagsBits != 0) {
@@ -824,15 +826,18 @@ class ResolverTask extends CompilerTask {
                 MessageKind.ILLEGAL_CONSTRUCTOR_MODIFIERS,
                 {'modifiers': mismatchedFlags});
           }
+          // ignore: UNDEFINED_GETTER
           if (member.modifiers.isConst) {
             constConstructors.add(member);
           }
         }
         if (member.isField) {
+          // ignore: UNDEFINED_GETTER
           if (member.modifiers.isConst && !member.modifiers.isStatic) {
             reporter.reportErrorMessage(
                 member, MessageKind.ILLEGAL_CONST_FIELD_MODIFIER);
           }
+          // ignore: UNDEFINED_GETTER
           if (!member.modifiers.isStatic && !member.modifiers.isFinal) {
             nonFinalInstanceFields.add(member);
           }