Tweaks to improve performance of type-inference.

pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
index a5680663b42babb1f0b732cb0e6a77ce72a21d3c..4ff72abcc81347b0ada2d56bd23603fe6c047361 100644
--- a/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/type_graph_inferrer.dart
@@ -538,11 +538,28 @@ class TypeInformationSystem extends TypeSystem<TypeInformation> {
   }
 
   TypeMask joinTypeMasks(Iterable<TypeMask> masks) {
-    TypeMask newType = const TypeMask.nonNullEmpty();
+    var dynamicType = compiler.typesTask.dynamicType;
+    // Optimization: we are iterating over masks twice, but because `masks` is a
+    // mapped iterable, we save the intermediate results to avoid computing them
+    // again.
+    var list = [];
     for (TypeMask mask in masks) {
-      newType = newType.union(mask, classWorld);
+      // Don't do any work on computing unions if we know that after all that
+      // work the result will be `dynamic`.
+      // TODO(sigmund): change to `mask == dynamicType` so we can continue to
+      // track the non-nullable bit.
+      if (mask.containsAll(classWorld)) return dynamicType;

[Siggi Cherem (dart-lang), 2015/12/17 01:02:57]

note: i reverted a change I did here: instead of `mask == dynamicType` I'm
checking `mask.containsAll(world)`. Turns out that doing == dyanmicType is more
precise (we can preserve non-null on joins).

I switched back to remain consistent with the previous semantics and keep this
CL just about optimizations. I added a TODO for now. I might send out a separate
CL changing that.

+      list.add(mask);
     }
-    return newType.containsAll(classWorld) ? dynamicType.type : newType;
+
+    TypeMask newType = null;
+    for (TypeMask mask in masks) {
+      newType = newType == null ? mask : newType.union(mask, classWorld);
+      // Likewise - stop early if we already reach dynamic.
+      if (newType.containsAll(classWorld)) return dynamicType;
+    }
+
+    return newType ?? const TypeMask.nonNullEmpty();
   }
 }