Fixes issues in string interpolation tests

frog/tokenizer.dart

Index: frog/tokenizer.dart
diff --git a/frog/tokenizer.dart b/frog/tokenizer.dart
index da7338438ff831420ac1bbc7e9ee807c6d723337..2cfdec6bf4fac2ea52f2858c3f9f99947f8aa3be 100644
--- a/frog/tokenizer.dart
+++ b/frog/tokenizer.dart
@@ -379,17 +379,28 @@ class TokenizerBase extends TokenizerHelpers {
     }
   }
 
-  Token finishIdentifier() {
-    while (_index < _text.length) {
-      if (!isIdentifierPart(_text.charCodeAt(_index++))) {
-        _index--;
-        break;
-      }
-    }
-    int kind = getIdentifierKind();
+  Token finishIdentifier(int ch) {
     if (_interpStack != null && _interpStack.depth == -1) {
       _interpStack.depth = 0;
+      if (ch == 36/*$*/) {
+        // illegal character after $ in string interpolation
+        return _errorToken();
+      }
+      while (_index < _text.length) {
+        if (!isInterpIdentifierPart(_text.charCodeAt(_index++))) {
+          _index--;
+          break;
+        }
+      }
+    } else {
+      while (_index < _text.length) {
+        if (!isIdentifierPart(_text.charCodeAt(_index++))) {
+          _index--;
+          break;
+        }
+      }
     }
+    int kind = getIdentifierKind();
     if (kind == TokenKind.IDENTIFIER) {
       return _finishToken(TokenKind.IDENTIFIER);
     } else {