Update handling of multiline strings to specification.

pkg/front_end/lib/src/fasta/quote.dart

Index: pkg/front_end/lib/src/fasta/quote.dart
diff --git a/pkg/front_end/lib/src/fasta/quote.dart b/pkg/front_end/lib/src/fasta/quote.dart
index 8d9525304f1d57a467dabc86573914029cafcecf..a0624144952c7ac9f67c17a7aacb6dad5b94138d 100644
--- a/pkg/front_end/lib/src/fasta/quote.dart
+++ b/pkg/front_end/lib/src/fasta/quote.dart
@@ -68,7 +68,7 @@ int lengthOfOptionalWhitespacePrefix(String first, int start) {
     }
     if (code == $TAB || code == $SPACE) continue;
     if (code == $CR) {
-      if (i + 1 < codeUnits.length && codeUnits[i] == $LF) {
+      if (i + 1 < codeUnits.length && codeUnits[i + 1] == $LF) {
         i++;
       }
       return i + 1;
@@ -140,17 +140,27 @@ String unescape(String string, Quote quote) {
   switch (quote) {
     case Quote.Single:
     case Quote.Double:
+      return !string.contains("\\")
+          ? string
+          : unescapeCodeUnits(string.codeUnits, false);
+
     case Quote.MultiLineSingle:
     case Quote.MultiLineDouble:
-      break;
+      return !string.contains("\\") && !string.contains("\r")
+          ? string
+          : unescapeCodeUnits(string.codeUnits, false);
 
     case Quote.RawSingle:
     case Quote.RawDouble:
+      return string;
+
     case Quote.RawMultiLineSingle:
     case Quote.RawMultiLineDouble:
-      return string;
+      return !string.contains("\r")
+          ? string
+          : unescapeCodeUnits(string.codeUnits, true);
   }
-  return !string.contains("\\") ? string : unescapeCodeUnits(string.codeUnits);
+  return internalError("Unhandled string quote: $quote");
 }
 
 const String incompleteSequence = "Incomplete escape sequence.";
@@ -161,7 +171,7 @@ const String invalidCodePoint = "Invalid code point.";
 
 // Note: based on
 // [StringValidator.validateString](pkg/compiler/lib/src/string_validator.dart).
-String unescapeCodeUnits(List<int> codeUnits) {
+String unescapeCodeUnits(List<int> codeUnits, bool isRaw) {
   // Can't use Uint8List or Uint16List here, the code units may be larger.
   List<int> result = new List<int>(codeUnits.length);
   int resultOffset = 0;
@@ -171,7 +181,12 @@ String unescapeCodeUnits(List<int> codeUnits) {
 
   for (int i = 0; i < codeUnits.length; i++) {
     int code = codeUnits[i];
-    if (code == $BACKSLASH) {
+    if (code == $CR) {
+      if (i + 1 < codeUnits.length && codeUnits[i + 1] == $LF) {
+        i++;
+      }
+      code = $LF;
+    } else if (!isRaw && code == $BACKSLASH) {
       if (codeUnits.length == ++i) return error(i, incompleteSequence);
       code = codeUnits[i];