Make ClosedWorld a `forTesting` property of ResolutionWorldBuilder.

tests/compiler/dart2js/type_combination_test.dart

Index: tests/compiler/dart2js/type_combination_test.dart
diff --git a/tests/compiler/dart2js/type_combination_test.dart b/tests/compiler/dart2js/type_combination_test.dart
index ab8b39d1e2b8bd6a09e6efd375c531544e0df2eb..479e7176630584fbad9b92690956c9371b52a2f0 100644
--- a/tests/compiler/dart2js/type_combination_test.dart
+++ b/tests/compiler/dart2js/type_combination_test.dart
@@ -102,7 +102,7 @@ class RuleSet {
 }
 
 void testUnion(MockCompiler compiler) {
-  ClosedWorld closedWorld = compiler.closedWorld;
+  ClosedWorld closedWorld = compiler.resolverWorld.closedWorldForTesting;
   RuleSet ruleSet = new RuleSet(
       'union', (t1, t2) => simplify(t1.union(t2, closedWorld), closedWorld));
   rule(type1, type2, result) => ruleSet.rule(type1, type2, result);
@@ -416,7 +416,7 @@ void testUnion(MockCompiler compiler) {
 void testIntersection(MockCompiler compiler) {
   JavaScriptBackend backend = compiler.backend;
   BackendHelpers helpers = backend.helpers;
-  ClosedWorld closedWorld = compiler.closedWorld;
+  ClosedWorld closedWorld = compiler.resolverWorld.closedWorldForTesting;
   RuleSet ruleSet =
       new RuleSet('intersection', (t1, t2) => t1.intersection(t2, closedWorld));
   rule(type1, type2, result) => ruleSet.rule(type1, type2, result);
@@ -726,7 +726,7 @@ void testIntersection(MockCompiler compiler) {
 }
 
 void testRegressions(MockCompiler compiler) {
-  ClosedWorld closedWorld = compiler.closedWorld;
+  ClosedWorld closedWorld = compiler.resolverWorld.closedWorldForTesting;
   TypeMask nonNullPotentialString =
       new TypeMask.nonNullSubtype(patternClass, closedWorld);
   Expect.equals(potentialString,
@@ -760,7 +760,7 @@ void main() {
         .registerTypeUse(new TypeUse.instantiation(patternImplClass.rawType));
     compiler.enqueuer.resolution.applyImpact(impactBuilder);
     compiler.closeResolution();
-    ClosedWorld closedWorld = compiler.closedWorld;
+    ClosedWorld closedWorld = compiler.resolverWorld.closedWorldForTesting;
 
     // Grab hold of a supertype for String so we can produce potential
     // string types.