Support string add in crankshaft:

src/arm/lithium-arm.h

Index: src/arm/lithium-arm.h
diff --git a/src/arm/lithium-arm.h b/src/arm/lithium-arm.h
index 6da7c8615ee1a00acd83487c65dbe85a0cdd47f1..8c3449a0fb2e4a1ebdf64efb3ffbbaeaca1e55ce 100644
--- a/src/arm/lithium-arm.h
+++ b/src/arm/lithium-arm.h
@@ -152,6 +152,7 @@ class LCodeGen;
   V(StoreKeyedSpecializedArrayElement)          \
   V(StoreNamedField)                            \
   V(StoreNamedGeneric)                          \
+  V(StringAdd)                                  \
   V(StringCharCodeAt)                           \
   V(StringCharFromCode)                         \
   V(StringLength)                               \
@@ -1706,6 +1707,22 @@ class LStoreKeyedSpecializedArrayElement: public LTemplateInstruction<0, 3, 0> {
 };
 
 
+class LStringAdd: public LTemplateInstruction<1, 2, 0> {
+ public:
+  LStringAdd(LOperand* left, LOperand* right) {
+    inputs_[0] = left;
+    inputs_[1] = right;
+  }
+
+  DECLARE_CONCRETE_INSTRUCTION(StringAdd, "string-add")
+  DECLARE_HYDROGEN_ACCESSOR(StringAdd)
+
+  LOperand* left() { return inputs_[0]; }
+  LOperand* right() { return inputs_[1]; }
+};
+
+
+
 class LStringCharCodeAt: public LTemplateInstruction<1, 2, 0> {
  public:
   LStringCharCodeAt(LOperand* string, LOperand* index) {