* String type inference using compiler framework.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index 5e73d4100d643e35aadda8cbd9ab4ed1c62d541a..030aedb0a8f2877ab7f8a93c03659211b5db4037 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -163,6 +163,20 @@ function ADD(x) {
 }
 
 
+// Left operand (this) is already a string.
+function STRING_ADD_LEFT(x) {
+  x = %ToString(%ToPrimitive(x, NO_HINT));
+  return %StringAdd(this, x);
+}
+
+
+// Right operand (x) is already a string.
+function STRING_ADD_RIGHT(x) {
+  var a = %ToString(%ToPrimitive(this, NO_HINT));
+  return %StringAdd(a, x);
+}
+
+
 // ECMA-262, section 11.6.2, page 50.
 function SUB(x) {
   return %NumberSub(%ToNumber(this), %ToNumber(x));
@@ -275,7 +289,7 @@ function IN(x) {
 
 
 // ECMA-262, section 11.8.6, page 54. To make the implementation more
-// efficient, the return value should be zero if the 'this' is an 
+// efficient, the return value should be zero if the 'this' is an
 // instance of F, and non-zero if not. This makes it possible to avoid
 // an expensive ToBoolean conversion in the generated code.
 function INSTANCE_OF(F) {