Implement script tags.

dart/frog/leg/elements/elements.dart

Index: dart/frog/leg/elements/elements.dart
diff --git a/dart/frog/leg/elements/elements.dart b/dart/frog/leg/elements/elements.dart
index 5193a3b9272a76e1bfb69032d8841b90c84f2b10..2ff66099d31aeb8c91c719f5e49a3f273ff2c3af 100644
--- a/dart/frog/leg/elements/elements.dart
+++ b/dart/frog/leg/elements/elements.dart
@@ -32,6 +32,7 @@ class ElementKind {
   static final ElementKind SETTER = const ElementKind('setter');
   static final ElementKind ABSTRACT_FIELD = const ElementKind('abstract_field');
   static final ElementKind LIBRARY = const ElementKind('library');
+  static final ElementKind PREFIX = const ElementKind('prefix');
 
   toString() => id;
 }
@@ -177,6 +178,17 @@ class LibraryElement extends CompilationUnitElement {
   }
 }
 
+class PrefixElement extends Element {
+  final LiteralString prefix;
+  final LibraryElement library;
+
+  PrefixElement(LiteralString prefix,
+                LibraryElement this.library,
+                Element enclosing)
+    : this.prefix = prefix,
+      super(prefix.dartString.source, ElementKind.PREFIX, enclosing);
+}
+
 class VariableElement extends Element {
   final VariableListElement variables;
   Expression cachedNode; // The send or the identifier in the variables list.