More fixes for missing interface targets when not in strong mode.

pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
index fcbf81920b6cfb44cd3d725718c7cda74d66c322..e5ae7cd8268d4410f31266ef6bce8c811bb80c3c 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inferrer.dart
@@ -354,13 +354,17 @@ abstract class TypeInferrerImpl extends TypeInferrer {
       var interfaceMember = findInterfaceMember(
           receiverType, propertyGet.name, propertyGet.fileOffset,
           silent: silent);
-      propertyGet.interfaceTarget = interfaceMember;
+      if (strongMode) {
+        propertyGet.interfaceTarget = interfaceMember;
+      }
       return interfaceMember;
     } else if (propertyGet is SuperPropertyGet) {
       var interfaceMember = findInterfaceMember(
           receiverType, propertyGet.name, propertyGet.fileOffset,
           silent: silent);
-      propertyGet.interfaceTarget = interfaceMember;
+      if (strongMode) {
+        propertyGet.interfaceTarget = interfaceMember;
+      }
       return interfaceMember;
     } else {
       throw internalError(
@@ -376,13 +380,17 @@ abstract class TypeInferrerImpl extends TypeInferrer {
       var interfaceMember = findInterfaceMember(
           receiverType, propertySet.name, propertySet.fileOffset,
           setter: true, silent: silent);
-      propertySet.interfaceTarget = interfaceMember;
+      if (strongMode) {
+        propertySet.interfaceTarget = interfaceMember;
+      }
       return interfaceMember;
     } else if (propertySet is SuperPropertySet) {
       var interfaceMember = findInterfaceMember(
           receiverType, propertySet.name, propertySet.fileOffset,
           setter: true, silent: silent);
-      propertySet.interfaceTarget = interfaceMember;
+      if (strongMode) {
+        propertySet.interfaceTarget = interfaceMember;
+      }
       return interfaceMember;
     } else {
       throw internalError(