Issue 4810. In case of import conflict explicitly report which library, with which prefix imported …

compiler/java/com/google/dart/compiler/resolver/Elements.java

Index: compiler/java/com/google/dart/compiler/resolver/Elements.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/Elements.java b/compiler/java/com/google/dart/compiler/resolver/Elements.java
index 53f8723649f93700f48e2d0c4b70101a12aeb8bc..075fa1d2d98f43997d0447bae43d5c8edeb647e6 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -20,10 +20,10 @@ import com.google.dart.compiler.ast.DartFunctionExpression;
 import com.google.dart.compiler.ast.DartFunctionTypeAlias;
 import com.google.dart.compiler.ast.DartIdentifier;
 import com.google.dart.compiler.ast.DartLabel;
-import com.google.dart.compiler.ast.DartObsoleteMetadata;
 import com.google.dart.compiler.ast.DartMethodDefinition;
 import com.google.dart.compiler.ast.DartNativeBlock;
 import com.google.dart.compiler.ast.DartNode;
+import com.google.dart.compiler.ast.DartObsoleteMetadata;
 import com.google.dart.compiler.ast.DartParameter;
 import com.google.dart.compiler.ast.DartSuperExpression;
 import com.google.dart.compiler.ast.DartTypeNode;
@@ -595,6 +595,38 @@ static FieldElementImplementation fieldFromNode(DartField node,
   }
 
   /**
+   * @return the full location of the given {@link Element} - library and unit.
+   */
+  public static String getLibraryUnitLocation(Element element) {
+    try {
+      StringBuilder sb = new StringBuilder();
+      // library URI
+      String libraryUri;
+      {
+        LibraryUnit libraryUnit = getDeclaringLibrary(element).getLibraryUnit();
+        libraryUri = libraryUnit.getSource().getUri().toString();
+        sb.append(libraryUri);
+      }
+      // unit URI
+      SourceInfo sourceInfo = element.getSourceInfo();
+      {
+        String unitUri = sourceInfo.getSource().getUri().toString();
+        if (!unitUri.equals(libraryUri)) {
+          sb.append(" ");
+          sb.append(unitUri);
+        }
+      }
+      // line
+      sb.append(":");
+      sb.append(sourceInfo.getLine());
+      // done
+      return sb.toString();
+    } catch (Throwable e) {
+      return "<exception>";
+    }
+  }
+
+  /**
    * @return the enclosing {@link ClassElement} (may be same if already given {@link ClassElement}),
    *         may be <code>null</code> if top level element.
    */