Added unit test for string containing escaped quotes.

pmd-dart/src/test/java/net/sourceforge/pmd/cpd/DartTokenizerTest.java

@@ -31,6 +31,7 @@ public class DartTokenizerTest extends AbstractTokenizerTest {
     public static Collection<Object[]> data() {
         return Arrays.asList(
                 new Object[] { "comment.dart", 5 },
+                new Object[] { "escaped_string.dart", 17 },
                 new Object[] { "increment.dart", 185 },
                 new Object[] { "imports.dart", 1 }
         );

pmd-dart/src/test/resources/net/sourceforge/pmd/cpd/escaped_string.dart

@@ -0,0 +1,4 @@
+var a = "a";
+var b = "b";
+var c = "c";
+var x = "$a(b: $b, c: \"$c\")";