Scanner cleanups: use isolates to allocate handles and zone objects

runtime/vm/scanner.h

Index: runtime/vm/scanner.h
===================================================================
--- runtime/vm/scanner.h	(revision 37052)
+++ runtime/vm/scanner.h	(working copy)
@@ -18,6 +18,7 @@
 class Array;
 class Library;
 class RawString;
+class ScanContext;
 class String;
 
 // A call to Scan() scans the source one token at at time.
@@ -87,15 +88,10 @@
                              const String** value);
 
  private:
+  friend class ScanContext;
+
   static const int kNumLowercaseChars = 26;
 
-  struct ScanContext {
-    ScanContext* next;
-    char string_delimiter;
-    bool string_is_multiline;
-    int  brace_level;
-  };
-
   struct KeywordTable {
     Token::Kind kind;
     const char* keyword_chars;