Add support for implicitly concatenating adjacent string literals.

frog/parser.dart

Index: frog/parser.dart
diff --git a/frog/parser.dart b/frog/parser.dart
index 3a6ab3d749714fb25bb572a8f9ed3bc9734523ff..4cb22ab8250a72ba142f3976f8a566d59fb8fcef 100644
--- a/frog/parser.dart
+++ b/frog/parser.dart
@@ -1115,11 +1115,8 @@ class Parser {
           Value.fromDouble(Math.parseDouble(t.text), t.span));
 
       case TokenKind.STRING:
-      var t = _next();
-      return _makeLiteral(Value.fromString(t.value, t.span));
-
       case TokenKind.STRING_PART:
-        return stringInterpolation();
+        return adjacentStrings();
 
       case TokenKind.LT:
         return finishTypedLiteral(start, false);
@@ -1138,6 +1135,27 @@ class Parser {
     }
   }
 
+  adjacentStrings() {
+    int start = _peekToken.start;
+    List<Expression> strings = [];
+    while (_peek() == TokenKind.STRING || _peek() == TokenKind.STRING_PART) {
+      Expression part = null;
+      if (_peek() == TokenKind.STRING) {
+        var t = _next();
+        part = _makeLiteral(Value.fromString(t.value, t.span));
+      } else {
+        part = stringInterpolation();
+      }
+      strings.add(part);
+    }
+    if (strings.length == 1) {
+      return strings[0];
+    } else {
+      assert(!strings.isEmpty());
+      return new StringConcatExpression(strings, _makeSpan(start));
+    }
+  }
+
   stringInterpolation() {
     int start = _peekToken.start;
     var pieces = new List<Expression>();