Make string juxtaposition combine properly with string interpolations.

frog/leg/tree/nodes.dart

Index: frog/leg/tree/nodes.dart
diff --git a/frog/leg/tree/nodes.dart b/frog/leg/tree/nodes.dart
index b05f1e1cc331cd85e07d9102237122d756ca9de7..54d989d168c9167b3bc84d35d2e4ca891825040b 100644
--- a/frog/leg/tree/nodes.dart
+++ b/frog/leg/tree/nodes.dart
@@ -726,6 +726,11 @@ class DartString implements Iterable<int> {
       new RawSourceDartString(source, length);
   factory DartString.escapedString(SourceString source, int length) =>
       new EscapedSourceDartString(source, length);
+  factory DartString.concat(DartString first, DartString second) {
+    if (first.isEmpty()) return second;
+    if (second.isEmpty()) return first;
+    return new ConsDartString(first, second);
+  }
   DartString();
   abstract int get length();
   bool isEmpty() => length == 0;
@@ -1254,21 +1259,28 @@ class StringInterpolationPart extends Node {
 }
 
 class LiteralStringJuxtaposition extends LiteralString {
-  // List of either StringLiteral or StringInterpolation.
-  final Link<Expression> literals;
+  // List of StringLiterals.
+  final Link<LiteralString> literals;
 
-  LiteralStringJuxtaposition(Link<Expression> literals)
+  LiteralStringJuxtaposition(Link<LiteralString> literals)
       : this.literals = literals,
         super(literals.head.getBeginToken(), concatenateLiterals(literals));
 
-  static DartString concatenateLiterals(Link<Expression> literals) {
+  static DartString concatenateLiterals(Link<LiteralString> literals) {
     assert(!literals.isEmpty());
     LiteralString literal = literals.head;
-    if (literals.tail.isEmpty()) {
-      return literal.dartString;
+    // If any of the literals couldn't be validated, then nor can their
+    // concatenation.
+    if (!literal.isValidated()) return null;
+    DartString accumulator = literal.dartString;
+    literals = literals.tail;
+    while (!literals.isEmpty()) {
+      literal = literals.head;
+      if (!literal.isValidated()) return null;
+      accumulator = new DartString.concat(accumulator, literal.dartString);
+      literals = literals.tail;
     }
-    return new ConsDartString(literal.dartString,
-                              concatenateLiterals(literals.tail));
+    return accumulator;
   }
 
   SourceString get value() => null;