Fixes and tests for adding library imports.

pkg/analyzer_plugin/lib/src/utilities/change_builder/change_builder_dart.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 1063 matching lines @@
     }
 
     // Prepare all URIs to import.
     List<String> uriList = libraries
         .map((library) => _getLibrarySourceUri(targetLibrary, library))
         .toList();
     uriList.sort((a, b) => a.compareTo(b));
 
     // Insert imports: between existing imports.
     if (importDirectives.isNotEmpty) {
-      bool isFirstPackage = true;
       for (String importUri in uriList) {
+        bool isDart = importUri.startsWith('dart:');
+        bool isPackage = importUri.startsWith('package:');
         bool inserted = false;
-        bool isPackage = importUri.startsWith('package:');
-        bool isAfterDart = false;
+
+        void insert(
+            {ImportDirective prev,
+            ImportDirective next,
+            String uri,
+            bool trailingNewLine: false}) {
+          if (prev != null) {
+            addInsertion(prev.end, (EditBuilder builder) {
+              builder.writeln();
+              builder.write("import '");
+              builder.write(uri);
+              builder.write("';");
+            });
+          } else {
+            addInsertion(next.offset, (EditBuilder builder) {
+              builder.write("import '");
+              builder.write(uri);
+              builder.writeln("';");
+              if (trailingNewLine) {
+                builder.writeln();
+              }
+            });
+          }
+          inserted = true;
+        }
+
+        ImportDirective lastExisting;
+        ImportDirective lastExistingDart;
+        ImportDirective lastExistingPackage;
+        bool isLastExistingDart = false;
+        bool isLastExistingPackage = false;
         for (ImportDirective existingImport in importDirectives) {
-          if (existingImport.uriContent.startsWith('dart:')) {
-            isAfterDart = true;
+          String existingUri = existingImport.uriContent;
+
+          bool isExistingDart = existingUri.startsWith('dart:');
+          bool isExistingPackage = existingUri.startsWith('package:');
+          bool isExistingRelative = !existingUri.contains(':');
+
+          bool isNewBeforeExisting = importUri.compareTo(existingUri) < 0;
+
+          if (isDart) {
+            if (!isExistingDart || isNewBeforeExisting) {
+              insert(
+                  prev: lastExistingDart,
+                  next: existingImport,
+                  uri: importUri,
+                  trailingNewLine: !isExistingDart);
+              break;
+            }
+          } else if (isPackage) {
+            if (isExistingRelative || isNewBeforeExisting) {
+              insert(
+                  prev: lastExistingPackage,
+                  next: existingImport,
+                  uri: importUri,
+                  trailingNewLine: isExistingRelative);
+              break;
+            }
+          } else {
+            if (!isExistingDart && !isExistingPackage && isNewBeforeExisting) {
+              insert(next: existingImport, uri: importUri);
+              break;
+            }
           }
-          if (existingImport.uriContent.startsWith('package:')) {
-            isFirstPackage = false;
+
+          lastExisting = existingImport;
+          if (isExistingDart) {
+            lastExistingDart = existingImport;
+          } else if (isExistingPackage) {
+            lastExistingPackage = existingImport;
           }
-          if (importUri.compareTo(existingImport.uriContent) < 0) {
-            addInsertion(existingImport.offset, (EditBuilder builder) {
-              builder.write("import '");
-              builder.write(importUri);
-              builder.writeln("';");
-            });
-            inserted = true;
-            break;
-          }
+          isLastExistingDart = isExistingDart;
+          isLastExistingPackage = isExistingPackage;
         }
         if (!inserted) {
-          addInsertion(importDirectives.last.end, (EditBuilder builder) {
-            if (isPackage && isFirstPackage && isAfterDart) {
-              builder.writeln();
+          addInsertion(lastExisting.end, (EditBuilder builder) {
+            if (isPackage) {
+              if (isLastExistingDart) {
+                builder.writeln();
+              }
+            } else {
+              if (isLastExistingDart || isLastExistingPackage) {
+                builder.writeln();
+              }
             }
             builder.writeln();
             builder.write("import '");
             builder.write(importUri);
             builder.write("';");
           });
         }
-        if (isPackage) {
-          isFirstPackage = false;
-        }
       }
       return;
     }
 
     // Insert imports: after the library directive.
     if (libraryDirective != null) {
       for (int i = 0; i < uriList.length; i++) {
         String importUri = uriList[i];
         addInsertion(libraryDirective.end, (EditBuilder builder) {
           if (i == 0) {
@@ skipping 212 matching lines @@
   }
 }
 
 class _InsertionDescription {
   final int offset;
   final bool insertEmptyLineBefore;
   final bool insertEmptyLineAfter;
   _InsertionDescription(
       this.offset, this.insertEmptyLineBefore, this.insertEmptyLineAfter);
 }