Return null from stringValue and call slowToString() instead of toString().

dart/frog/leg/ssa/builder.dart

Index: dart/frog/leg/ssa/builder.dart
diff --git a/dart/frog/leg/ssa/builder.dart b/dart/frog/leg/ssa/builder.dart
index d4b429f7cde76aceb76d7eedb2c8dfd1570584b1..ef0e5e940ca4825ea5f9230b8646654a8f5d0725 100644
--- a/dart/frog/leg/ssa/builder.dart
+++ b/dart/frog/leg/ssa/builder.dart
@@ -44,19 +44,19 @@ class Interceptors {
   }
 
   Element getStaticInterceptor(SourceString name, int parameters) {
-    String mangledName = "builtin\$${name}\$${parameters}";
+    String mangledName = "builtin\$${name.slowToString()}\$${parameters}";
     Element result = compiler.findHelper(new SourceString(mangledName));
     return result;
   }
 
   Element getStaticGetInterceptor(SourceString name) {
-    String mangledName = "builtin\$get\$${name}";
+    String mangledName = "builtin\$get\$${name.slowToString()}";
     Element result = compiler.findHelper(new SourceString(mangledName));
     return result;
   }
 
   Element getStaticSetInterceptor(SourceString name) {
-    String mangledName = "builtin\$set\$${name}";
+    String mangledName = "builtin\$set\$${name.slowToString()}";
     Element result = compiler.findHelper(new SourceString(mangledName));
     return result;
   }
@@ -1196,7 +1196,7 @@ class SsaBuilder implements Visitor {
   }
 
   SourceString unquote(LiteralString literal, int start) {
-    String str = '${literal.value}';
+    String str = '${literal.value.slowToString()}';
     int quotes = 1;
     String quote = str[start];
     while (str[quotes + start] === quote) quotes++;
@@ -1662,7 +1662,7 @@ class SsaBuilder implements Visitor {
 
   visitForeignSend(Send node) {
     Identifier selector = node.selector;
-    switch (selector.source.stringValue) {
+    switch (selector.source.slowToString()) {
       case "JS":
         Link<Node> link = node.arguments;
         // If the invoke is on foreign code, don't visit the first
@@ -1675,7 +1675,7 @@ class SsaBuilder implements Visitor {
         LiteralString literal = node.arguments.tail.head;
         compiler.ensure(literal is LiteralString);
         compiler.ensure(type is LiteralString);
-        compiler.ensure(literal.value.stringValue[0] == '@');
+        compiler.ensure(literal.value.slowToString()[0] == '@');
         push(new HForeign(unquote(literal, 1), unquote(type, 0), inputs));
         break;
       case "UNINTERCEPTED":