All strong mode cleaning of dart2js.

pkg/compiler/lib/src/types/union_type_mask.dart

Index: pkg/compiler/lib/src/types/union_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/union_type_mask.dart b/pkg/compiler/lib/src/types/union_type_mask.dart
index 1997d0275502b1d187f8f24159ad7ce82539da9f..b5ace962914fe4e0fff5bdb9d61424a9c9ba7a87 100644
--- a/pkg/compiler/lib/src/types/union_type_mask.dart
+++ b/pkg/compiler/lib/src/types/union_type_mask.dart
@@ -189,8 +189,10 @@ class UnionTypeMask implements TypeMask {
 
   TypeMask nonNullable() {
     if (!isNullable) return this;
-    Iterable<FlatTypeMask> newIterable =
-        disjointMasks.map((e) => e.nonNullable());
+    Iterable<FlatTypeMask> newIterable = disjointMasks.map((e) {
+      FlatTypeMask r = e.nonNullable();
+      return r;
+    });
     return new UnionTypeMask._internal(newIterable);
   }