dart2js: improve performance of TypeMask.intersection

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

Index: pkg/compiler/lib/src/types/flat_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/flat_type_mask.dart b/pkg/compiler/lib/src/types/flat_type_mask.dart
index 0f3aa13bca90da8ee0acfc5c6584e0f56ae4af41..270271d1ae88de617b4665984313cbb0c8a5958c 100644
--- a/pkg/compiler/lib/src/types/flat_type_mask.dart
+++ b/pkg/compiler/lib/src/types/flat_type_mask.dart
@@ -732,16 +732,7 @@ class FlatTypeMask implements TypeMask {
     if (xSubset == null) return null;
     Iterable<ClassElement> ySubset = containedSubset(y, classWorld);
     if (ySubset == null) return null;
-    Iterable<ClassElement> smallSet, largeSet;
-    if (xSubset.length <= ySubset.length) {

[Siggi Cherem (dart-lang), 2016/01/21 01:07:12]

this was the most important change: this was worse case quadratic (the iterable
was traverse here twice, then n times on each contains check below).

-      smallSet = xSubset;
-      largeSet = ySubset;
-    } else {
-      smallSet = ySubset;
-      largeSet = xSubset;
-    }
-    var result = smallSet.where((ClassElement each) => largeSet.contains(each));
-    return result.toSet();
+    return xSubset.toSet().intersection(ySubset.toSet());
   }
 
   static Iterable<ClassElement> containedSubset(FlatTypeMask x,