dart2js: Avoid escaping in strings if it's not necessary.

pkg/compiler/lib/src/js/builder.dart

Index: pkg/compiler/lib/src/js/builder.dart
diff --git a/pkg/compiler/lib/src/js/builder.dart b/pkg/compiler/lib/src/js/builder.dart
index 5d8da87f88b6f1de2f453fef6bdced2bf29cbec8..37ef7d10c01471fe955980f46d52fdf84aa72d77 100644
--- a/pkg/compiler/lib/src/js/builder.dart
+++ b/pkg/compiler/lib/src/js/builder.dart
@@ -289,20 +289,47 @@ class JsBuilder {
     return new Template.withStatementResult(ast);
   }
 
+  static RegExp _stringEscapeRegExp =
+      new RegExp(r'["\\\n\r\b\t\v' + '\u2028\u2029]');
+
   /// Creates a literal js string from [value].
   LiteralString escapedString(String value) {
-   // Start by escaping the backslashes.
-    String escaped = value.replaceAll('\\', '\\\\');
+    // Relevant sections of ECMA-262:
+    //
+    //     7.3 Line Terminators
+    //       LineTerminator ::
+    //          <LF>
+    //          <CR>
+    //          <LS>
+    //          <PS>
+    //
+    //     7.8.4 String Literals
+    //       StringLiteral ::
+    //          " DoubleStringCharacters? "
+    //          ' SingleStringCharacters? '
+    //
+    //       DoubleStringCharacters ::
+    //          DoubleStringCharacter DoubleStringCharacters?
+    //
+    //       DoubleStringCharacter ::
+    //          SourceCharacter but not one of " or \ or LineTerminator
+    //          \ EscapeSequence
+    //          LineContinuation
+    //
     // Do not escape unicode characters and ' because they are allowed in the
     // string literal anyway.
-    escaped = escaped.replaceAllMapped(new RegExp('\n|"|\b|\t|\v'), (match) {
+    String escaped = value.replaceAllMapped(_stringEscapeRegExp, (Match match) {
       switch (match.group(0)) {
+        case "\\" : return r"\\";
         case "\n" : return r"\n";
+        case "\r" : return r"\r";
         case "\"" : return r'\"';
         case "\b" : return r"\b";
         case "\t" : return r"\t";
         case "\f" : return r"\f";
         case "\v" : return r"\v";
+        case "\u2028" : return r"\u2028";
+        case "\u2029" : return r"\u2029";
       }
     });
     LiteralString result = string(escaped);