[strong] Simplify (and sortof optimize) string addition for strong mode.

src/runtime.js

Index: src/runtime.js
diff --git a/src/runtime.js b/src/runtime.js
index a120d9648c494b1d586393a10d053912e09bd2d3..e82d25e3295c745af5ee701e1166f9fdc5935aea 100644
--- a/src/runtime.js
+++ b/src/runtime.js
@@ -23,9 +23,7 @@ var COMPARE_STRONG;
 var ADD;
 var ADD_STRONG;
 var STRING_ADD_LEFT;
-var STRING_ADD_LEFT_STRONG;
 var STRING_ADD_RIGHT;
-var STRING_ADD_RIGHT_STRONG;
 var SUB;
 var SUB_STRONG;
 var MUL;
@@ -262,15 +260,6 @@ STRING_ADD_LEFT = function STRING_ADD_LEFT(y) {
 }
 
 
-// Left operand (this) is already a string.
-STRING_ADD_LEFT_STRONG = function STRING_ADD_LEFT_STRONG(y) {
-  if (IS_STRING(y)) {
-    return %_StringAdd(this, y);
-  }
-  throw %MakeTypeError(kStrongImplicitConversion);
-}
-
-
 // Right operand (y) is already a string.
 STRING_ADD_RIGHT = function STRING_ADD_RIGHT(y) {
   var x = this;
@@ -287,15 +276,6 @@ STRING_ADD_RIGHT = function STRING_ADD_RIGHT(y) {
 }
 
 
-// Right operand (y) is already a string.
-STRING_ADD_RIGHT_STRONG = function STRING_ADD_RIGHT_STRONG(y) {
-  if (IS_STRING(this)) {
-    return %_StringAdd(this, y);
-  }
-  throw %MakeTypeError(kStrongImplicitConversion);
-}
-
-
 // ECMA-262, section 11.6.2, page 50.
 SUB = function SUB(y) {
   var x = IS_NUMBER(this) ? this : %$nonNumberToNumber(this);