Allow initializing a field in a superclass in a const constructor.

compiler/java/com/google/dart/compiler/resolver/ClassElementImplementation.java

Index: compiler/java/com/google/dart/compiler/resolver/ClassElementImplementation.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/ClassElementImplementation.java b/compiler/java/com/google/dart/compiler/resolver/ClassElementImplementation.java
index 3e6b2ae5cdd577acd1b726919851b33d9d09dd59..8f730ba3f4c17af28ccebfaf0ffa2727b942e20a 100644
--- a/compiler/java/com/google/dart/compiler/resolver/ClassElementImplementation.java
+++ b/compiler/java/com/google/dart/compiler/resolver/ClassElementImplementation.java
@@ -268,6 +268,33 @@ class ClassElementImplementation extends AbstractElement implements ClassElement
     return null;
   }
 
+  /**
+   * Looks through all the fields in this class and its supertypes. Look through supertypes
+   * first, so that we can use this method to find conflicts.
+   */
+  FieldElement lookupField(String name) {
+    // Look through super classes
+    try {
+      if (!isInterface()) {
+        for (InterfaceType type : getAllSupertypes()) {
+          if (!type.getElement().isInterface()) {
+            InterfaceType.Member member = type.lookupMember(name);
+            if (member != null) {
+              FieldElement fieldElement = (FieldElement)member.getElement();
+              if (ElementKind.of(fieldElement).equals(ElementKind.FIELD) &&
+                  !fieldElement.getModifiers().isAbstract()) {
+                return (FieldElement) member.getElement();
+              }
+            }
+          }
+        }
+      }
+    } catch (CyclicDeclarationException ignored) {
+    } catch (DuplicatedInterfaceException ignored) {
+    }
+    return lookupLocalField(name);
+  }
+
   MethodElement lookupLocalMethod(String name) {
     Element element = lookupLocalElement(name);
     if (ElementKind.of(element).equals(ElementKind.METHOD)) {