[dart] Add multi-line string example

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

@@ -5,6 +5,7 @@
 package net.sourceforge.pmd.cpd;
 
 import java.io.IOException;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.Collection;
 
@@ -39,7 +40,8 @@ public class DartTokenizerTest extends AbstractTokenizerTest {
                 new Object[] { "regex.dart", 13 },
                 new Object[] { "regex2.dart", 13 },
                 new Object[] { "regex3.dart", 13 },
-                new Object[] { "string_with_backslashes.dart", 9 }
+                new Object[] { "string_with_backslashes.dart", 9 },
+                new Object[] { "string_multiline.dart", 13 }
         );
     }
 
@@ -52,7 +54,7 @@ public class DartTokenizerTest extends AbstractTokenizerTest {
 
     @Override
     public String getSampleCode() throws IOException {
-        return IOUtils.toString(DartTokenizer.class.getResourceAsStream(this.filename));
+        return IOUtils.toString(DartTokenizer.class.getResourceAsStream(this.filename), StandardCharsets.UTF_8);
     }
 
     @Test

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

@@ -0,0 +1,9 @@
+class MyClass {
+    var s1 = '''
+You can create
+multi-line strings like this one.
+''';
+
+    var s2 = """This is also a
+multi-line string.""";
+}