Handle external strings in generated code when concatenating short strings.

test/mjsunit/string-external-cached.js

Index: test/mjsunit/string-external-cached.js
diff --git a/test/mjsunit/string-external-cached.js b/test/mjsunit/string-external-cached.js
index 0a95830d0b7824b9cd6f1f046831d52f638749bb..6e24285331cca3c25a424d3d7d17666c3e643556 100644
--- a/test/mjsunit/string-external-cached.js
+++ b/test/mjsunit/string-external-cached.js
@@ -86,6 +86,29 @@ function test() {
     assertEquals("DEFG", ascii_cons.substr(3, 4));
     assertEquals("DEFG", twobyte_cons.substr(4, 4));
   }
+
+  // Test adding external strings
+  var short_ascii = "E=";
+  var long_ascii = "MCsquared";
+  var short_twobyte = "E\u1234";
+  var long_twobyte = "MCsquare\u1234";
+  try {  // String can only be externalized once
+    externalizeString(short_ascii, false);
+    externalizeString(long_ascii, false);
+    externalizeString(short_twobyte, true);
+    externalizeString(long_twobyte, true);
+    assertTrue(isAsciiString(short_asii) && isAsciiString(long_ascii));
+    assertFalse(isAsciiString(short_twobyte) || isAsciiString(long_twobyte));
+  } catch (ex) { }
+  assertEquals("E=MCsquared", short_ascii + long_ascii);
+  assertTrue(isAsciiString(short_ascii + long_ascii));
+  assertEquals("MCsquaredE=", long_ascii + short_ascii);
+  assertEquals("E\u1234MCsquare\u1234", short_twobyte + long_twobyte);
+  assertFalse(isAsciiString(short_twobyte + long_twobyte));
+  assertEquals("E=MCsquared", "E=" + long_ascii);
+  assertEquals("E\u1234MCsquared", short_twobyte + "MCsquared");
+  assertEquals("E\u1234MCsquared", short_twobyte + long_ascii);
+  assertFalse(isAsciiString(short_twobyte + long_ascii));
 }
 
 // Run the test many times to ensure IC-s don't break things.