Fix JSON.stringifier's slow path wrt external strings.

test/mjsunit/json2.js

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-// Flags: --allow-natives-syntax
+// Flags: --allow-natives-syntax --expose-externalize-string
 
 // Test JSON.stringify on the global object.
 var a = 12345;
 assertTrue(JSON.stringify(this).indexOf('"a":12345') > 0);
 assertTrue(JSON.stringify(this, null, 0).indexOf('"a":12345') > 0);
 
 // Test JSON.stringify of array in dictionary mode.
 function TestStringify(expected, input) {
   assertEquals(expected, JSON.stringify(input));
   assertEquals(expected, JSON.stringify(input, null, 0));
@@ skipping 126 matching lines @@
 assertEquals('{"a":1,"b":2,"d":4}',
              JSON.stringify(getter_side_effect, null, 0));
 assertEquals('{"b":2,"d":4,"e":5}',
              JSON.stringify(getter_side_effect, null, 0));
 
 var non_enum = {};
 non_enum.a = 1;
 Object.defineProperty(non_enum, "b", { value: 2, enumerable: false });
 non_enum.c = 3;
 TestStringify('{"a":1,"c":3}', non_enum);
+
+var str = "external";
+try {
+  externalizeString(str, true);
+} catch (e) { }
+TestStringify("\"external\"", str, null, 0);