improve fasta unterminated string recovery

pkg/compiler/lib/src/string_validator.dart

Index: pkg/compiler/lib/src/string_validator.dart
diff --git a/pkg/compiler/lib/src/string_validator.dart b/pkg/compiler/lib/src/string_validator.dart
index 75f59f54a81352479f999f2239cc5b80834047fb..208353a52f5d666390dfe573a025c2df782a4445 100644
--- a/pkg/compiler/lib/src/string_validator.dart
+++ b/pkg/compiler/lib/src/string_validator.dart
@@ -25,7 +25,13 @@ class StringValidator {
     int leftQuote = 0;
     int rightQuote = 0;
     if (isFirst) leftQuote = quoting.leftQuoteLength;
-    if (isLast) rightQuote = quoting.rightQuoteLength;
+    if (isLast) {
+      // Check for unterminated string
+      if (leftQuote < source.length &&
+          source[source.length - 1] == quoting.quoteChar) {
+        rightQuote = quoting.rightQuoteLength;

[ahe, 2017/05/29 15:05:30]

What happens with this string:

"""x"

+      }
+    }
     String content = copyWithoutQuotes(source, leftQuote, rightQuote);
     return validateString(
         token, token.charOffset + leftQuote, content, quoting);