Add support for implicitly concatenating adjacent string literals.

frog/tree.g.dart

Index: frog/tree.g.dart
diff --git a/frog/tree.g.dart b/frog/tree.g.dart
index 0ddd12427ccb3d0f6e67dd8a259777a2fd45070b..3a398bd46166637122a719d2fcc03a41b39bb66d 100644
--- a/frog/tree.g.dart
+++ b/frog/tree.g.dart
@@ -369,6 +369,14 @@ class LiteralExpression extends Expression {
   visit(TreeVisitor visitor) => visitor.visitLiteralExpression(this);
 }
 
+class StringConcatExpression extends Expression {
+  List<Expression> strings;
+
+  StringConcatExpression(this.strings, SourceSpan span): super(span) {}
+
+  visit(TreeVisitor visitor) => visitor.visitStringConcatExpression(this);
+}
+
 class StringInterpExpression extends Expression {
   List<Expression> pieces;
 
@@ -574,6 +582,8 @@ interface TreeVisitor {
 
   visitLiteralExpression(LiteralExpression node);
 
+  visitStringConcatExpression(StringConcatExpression node);
+
   visitStringInterpExpression(StringInterpExpression node);
 
   visitSimpleTypeReference(SimpleTypeReference node);
@@ -847,6 +857,11 @@ class TreePrinter implements TreeVisitor {
     output.heading('LiteralExpression(' + output.toValue(node.value) + ")", node.span);
   }
 
+  void visitStringConcatExpression(StringConcatExpression node) {
+    output.heading('StringConcatExpression', node.span);
+    output.writeNodeList('strings', node.strings);
+  }
+
   void visitStringInterpExpression(StringInterpExpression node) {
     output.heading('StringInterpExpression', node.span);
     output.writeNodeList('pieces', node.pieces);