Change signature of lookupConstructor to only require a name.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index 0531a73146c188b08eaa188d423d28b1fa93db1f..631ed38c0388d5fec92b2d4af9a50a868a86a1db 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -2400,24 +2400,22 @@ abstract class BaseClassElementX extends ElementX
     return false;
   }
 
-  Element validateConstructorLookupResults(Selector selector,
-                                           Element result,
-                                           Element noMatch(Element)) {
-    if (result == null
-        || !result.isConstructor
-        || (isPrivateName(selector.name)
-            && result.library != selector.library)) {
-      result = noMatch != null ? noMatch(result) : null;
+  ConstructorElement lookupDefaultConstructor() {
+    ConstructorElement constructor = lookupConstructor("");
+    // This method might be called on constructors that have not been
+    // resolved. As we query the live world, we return `null` in such cases
+    // as no default constructor exists in the live world.
+    if (constructor != null &&
+        constructor.hasFunctionSignature &&
+        constructor.functionSignature.requiredParameterCount == 0) {
+      return constructor;
     }
-    return result;
+    return null;
   }
 
-  // TODO(aprelev@gmail.com): Peter believes that it would be great to
-  // make noMatch a required argument. Peter's suspicion is that most
-  // callers of this method would benefit from using the noMatch method.
-  Element lookupConstructor(Selector selector, [Element noMatch(Element)]) {
-    Element result = localLookup(selector.name);
-    return validateConstructorLookupResults(selector, result, noMatch);
+  ConstructorElement lookupConstructor(String name) {
+    Element result = localLookup(name);
+    return result != null && result.isConstructor ? result : null;
   }
 
   Link<Element> get constructors {