Refactor Library/CompilationUnit and how we define local Scope.

lib/compiler/implementation/js_backend/backend.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.
 
 class InvocationInfo {
   int parameterCount;
   List<HType> providedTypes;
   List<Element> compiledFunctions;
 
   InvocationInfo(List<HType> types)
@@ skipping 128 matching lines @@
   // that could alter the field.
   HType optimisticFieldTypeAfterConstruction(Element field) {
     assert(field.isField());
     assert(field.isMember());
 
     ClassElement classElement = field.getEnclosingClass();
     if (hasConstructorBodyFieldSetter(field)) {
       // If there are field setters but there is only constructor then the type
       // of the field is determined by the assignments in the constructor
       // body.
-      if (classElement.constructors.length == 1) {
+      var constructors = classElement.constructors;
+      if (constructors.head !== null && constructors.tail === null) {
         return fieldConstructorSetters[classElement][field];
       } else {
         return HType.UNKNOWN;
       }
     } else if (fieldInitializers.containsKey(classElement)) {
       HType type = fieldInitializers[classElement][field];
       return type == null ? HType.CONFLICTING : type;
     } else {
       return HType.CONFLICTING;
     }
@@ skipping 110 matching lines @@
     if (foundCount == 1 && found.hasTypeInformation) {
       FunctionSignature signature = element.computeSignature(compiler);
       if (signature.parameterCount == found.parameterCount) {
         found.addCompiledFunction(element);
         return found.providedTypes;
       }
     }
     return null;
   }
 }