Element model cleanup + prepare for easier reimplementation.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 65d269cb85c32d5bfd6c39cf2e3f5805f38b99a4..a81be046d9dcd216349423a59870c8643ee5ba82 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1927,11 +1927,14 @@ abstract class Compiler implements DiagnosticListener {
   }
 
   SourceSpan spanFromElement(Element element) {
+    if (element != null && element.sourcePosition != null) {
+      return element.sourcePosition;
+    }
     while (element != null && element.isSynthesized) {
       element = element.enclosingElement;
     }
     if (element != null &&
-        element.position == null &&
+        element.sourcePosition == null &&
         !element.isLibrary &&
         !element.isCompilationUnit) {
       // Sometimes, the backend fakes up elements that have no
@@ -1945,6 +1948,9 @@ abstract class Compiler implements DiagnosticListener {
     if (element == null) {
       element = currentElement;
     }
+    if (element.sourcePosition != null) {
+      return element.sourcePosition;
+    }
     Token position = element.position;
     Uri uri = element.compilationUnit.script.readableUri;
     return (position == null)