Add some support for the code-point code-unit distinction.

runtime/vm/scanner.cc

Index: runtime/vm/scanner.cc
diff --git a/runtime/vm/scanner.cc b/runtime/vm/scanner.cc
index 229dfdf36f0a9b06d37104a6ec56f31b1283bff0..849524559c114b8c3165c96bca78c43eae2eae44 100644
--- a/runtime/vm/scanner.cc
+++ b/runtime/vm/scanner.cc
@@ -162,11 +162,11 @@ bool Scanner::IsIdent(const String& str) {
   if (!str.IsOneByteString()) {
     return false;
   }
-  if (str.Length() == 0 || !IsIdentStartChar(str.CharAt(0))) {
+  if (str.Length() == 0 || !IsIdentStartChar(str.CodeUnitAt(0))) {
     return false;
   }
   for (int i =  1; i < str.Length(); i++) {
-    if (!IsIdentChar(str.CharAt(i))) {
+    if (!IsIdentChar(str.CodeUnitAt(i))) {
       return false;
     }
   }
@@ -180,7 +180,7 @@ void Scanner::ReadChar() {
       newline_seen_ = true;
       c0_pos_.line++;
       c0_pos_.column = 0;
-      if (source_.CharAt(lookahead_pos_) == '\r') {
+      if (source_.CodeUnitAt(lookahead_pos_) == '\r') {
         // Replace a sequence of '\r' '\n' with a single '\n'.
         if (LookaheadChar(1) == '\n') {
           lookahead_pos_++;
@@ -205,7 +205,7 @@ int32_t Scanner::LookaheadChar(int how_many) {
   ASSERT(how_many >= 0);
   int32_t lookahead_char = '\0';
   if (lookahead_pos_ + how_many < source_length_) {
-    lookahead_char = source_.CharAt(lookahead_pos_ + how_many);
+    lookahead_char = source_.CodeUnitAt(lookahead_pos_ + how_many);
   }
   return lookahead_char;
 }
@@ -260,7 +260,7 @@ void Scanner::ScanIdentChars(bool allow_dollar) {
   ASSERT(allow_dollar || (c0_ != '$'));
   int ident_length = 0;
   int ident_pos = lookahead_pos_;
-  int32_t ident_char0 = source_.CharAt(ident_pos);
+  int32_t ident_char0 = source_.CodeUnitAt(ident_pos);
   while (IsIdentChar(c0_) && (allow_dollar || (c0_ != '$'))) {
     ReadChar();
     ident_length++;
@@ -275,7 +275,8 @@ void Scanner::ScanIdentChars(bool allow_dollar) {
       const char* keyword = keywords_[i].keyword_chars;
       int char_pos = 0;
       while ((char_pos < ident_length) &&
-             (keyword[char_pos] == source_.CharAt(ident_pos + char_pos))) {
+             (static_cast<uint32_t>(keyword[char_pos]) ==
+              source_.CodeUnitAt(ident_pos + char_pos))) {
         char_pos++;
       }
       if (char_pos == ident_length) {