Stop reporting duplicate diagnostics for other ambiguous extensions

Change-Id: Ia81c43f0ccd41aa09697b6f629fd1d5a3acd3923
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/112643
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: bwilkerson

pkg/analyzer/lib/src/dart/resolver/method_invocation_resolver.dart

@@ -746,6 +746,8 @@ class MethodInvocationResolver {
             type, _nameCall.name, node.methodName, ElementKind.METHOD);
         if (result.isSingle) {
           call = result.element;
+        } else if (result.isAmbiguous) {
+          return;
         }
       }
       if (call != null && call.kind == ElementKind.METHOD) {

pkg/analyzer/lib/src/dart/resolver/resolution_result.dart

@@ -23,7 +23,9 @@ class ResolutionResult {
 
   /// Initialize a newly created result to represent resolving to a single
   /// [element].
-  const ResolutionResult(this.element) : state = _ResolutionResultState.single;
+  ResolutionResult(this.element)
+      : assert(element != null),
+        state = _ResolutionResultState.single;
 
   /// Initialize a newly created result with no element and the given [state].
   const ResolutionResult._(this.state) : element = null;