Don't constant fold large strings.

pkg/compiler/lib/src/cps_ir/type_propagation.dart

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index f716f05e023f7479bd72b9f731accc4b9f2a9dcb..c787e92b3bcad362977fb54143181adbd06cbc49 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -2541,7 +2541,20 @@ class TypePropagationVisitor implements Visitor {
                  {bool canReplace: false}) {
     // TODO(asgerf): Separate constant folding from side effect analysis.
     setValue(prim, updateValue);
-    prim.isSafeForElimination = canReplace && updateValue.isConstant;
+    prim.isSafeForElimination = canReplace &&
+        updateValue.isConstant &&
+        constantIsSafeToCopy(updateValue.constant);
+  }
+
+  bool constantIsSafeToCopy(ConstantValue constant) {
+    // TODO(25230, 25231): Refer to large shared strings by name.
+    // This number is chosen to prevent huge strings being copied. Don't make
+    // this too small, otherwise it will suppress otherwise harmless constant
+    // folding.
+    const int MAXIMUM_LENGTH_OF_DUPLICATED_STRING = 500;

[asgerf, 2015/12/11 12:27:12]

I think the better place to do this is in visitLetPrim above (I put a comment
there).  The operation itself should still be eliminated if it has no uses.

[sra1, 2015/12/11 19:13:26]

Done.

+    return constant is StringConstantValue
+        ? constant.length < MAXIMUM_LENGTH_OF_DUPLICATED_STRING
+        : true;
   }
 
   bool isInterceptedSelector(Selector selector) {