Cache model elements for compiler elements.

editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/index/contributor/IndexContributor.java

Index: editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/index/contributor/IndexContributor.java
diff --git a/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/index/contributor/IndexContributor.java b/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/index/contributor/IndexContributor.java
index bf23137be24ad06d3dedeada6cd47c67fc8d9a34..cc1689405cdb22c0299b9f3b6f3876c6137a6399 100644
--- a/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/index/contributor/IndexContributor.java
+++ b/editor/tools/plugins/com.google.dart.tools.core/src/com/google/dart/tools/core/internal/index/contributor/IndexContributor.java
@@ -166,6 +166,8 @@ public class IndexContributor extends ASTVisitor<Void> {
    */
   private IntStack unnamedFunctionCount = new IntStack();
 
+  private DartImport[] libraryImports;
+
   /**
    * Initialize a newly created contributor to contribute data and relationships to the given index
    * while processing the AST structure associated with the given compilation unit.
@@ -182,6 +184,7 @@ public class IndexContributor extends ASTVisitor<Void> {
     this.compilationUnit = compilationUnit;
     libraryResource = getResource(library.getDefiningCompilationUnit());
     libraryElement = new Element(libraryResource, LIBRARY_ELEMENT_ID);
+    libraryImports = library.getImports();
     compilationUnitResource = getResource(compilationUnit);
     compilationUnitElement = new Element(
         compilationUnitResource,
@@ -1189,14 +1192,12 @@ public class IndexContributor extends ASTVisitor<Void> {
   private void recordImportReference(LibraryElement importLibraryElement, String prefix, int offset) {
     try {
       DartLibrary importLibraryModel = BindingUtils.getDartElement(importLibraryElement);
-      CompilationUnit enclosingLibraryUnit = library.getDefiningCompilationUnit();
-      Resource enclosingLibraryResource = ResourceFactory.getResource(enclosingLibraryUnit);
-      for (DartImport imprt : library.getImports()) {
+      for (DartImport imprt : libraryImports) {
         if (Objects.equal(imprt.getLibrary(), importLibraryModel)
             && Objects.equal(imprt.getPrefix(), prefix)) {
           String imprtId = ElementFactory.composeElementId(imprt.getPrefix() + ":"
               + imprt.getLibrary().getElementName());
-          Element imprtElement = new Element(enclosingLibraryResource, imprtId);
+          Element imprtElement = new Element(libraryResource, imprtId);
           int length = StringUtils.length(prefix);
           Location location = createLocation(offset, length);
           recordRelationship(imprtElement, IndexConstants.IS_REFERENCED_BY, location);