Escaped characters in Dart multiline string are now handled correctly

pmd-dart/src/main/antlr4/net/sourceforge/pmd/lang/dart/antlr4/Dart2.g4

@@ -373,6 +373,7 @@ MultiLineString
 fragment
 StringContentTDQ
   : ~('\\' | '"' | '$')
+  | '\\' ~('\n' | '\r')
   | '"' ~'"' | '""' ~'"'
   | StringInterpolation
   ;
@@ -380,6 +381,7 @@ StringContentTDQ
 fragment
 StringContentTSQ
   : ~('\\' | '\'' | '$')
+  | '\\' ~('\n' | '\r')
   | '\'' ~'\'' | '\'\'' ~'\''
   | StringInterpolation
   ;

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

@@ -59,6 +59,11 @@ public class DartTokenizerTest extends CpdTextComparisonTest {
         doTest("string_interpolation");
     }
 
+    @Test
+    public void testEscapedDollar() {
+        doTest("escaped_dollar");
+    }
+
     @Test
     public void testRegex() {
         doTest("regex");

pmd-dart/src/test/resources/net/sourceforge/pmd/cpd/testdata/escaped_dollar.dart

@@ -0,0 +1,3 @@
+var multiLineStringWithDollar = """
+      \$
+      """;

pmd-dart/src/test/resources/net/sourceforge/pmd/cpd/testdata/escaped_dollar.txt

@@ -0,0 +1,7 @@
+    [Image] or [Truncated image[            Bcol      Ecol
+L1
+    [var]                                   1         3
+    [multiLineStringWithDollar]             5         29
+    [=]                                     31        31
+    ["""\n      \\$\n      """]             33        9
+EOF