AutoAssist in Android Studio for flutter List.length property mixed with List.length() method

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## Steps to Reproduce
List strings=["hi"];
int i = strings.l

After typing "strings.l" hit ctrl-space to bring up the auto-assist window.
The entry that is highlighted has a "P" icon (indicating a property), but shows the method signature "length (int newLength)".
It looks like the AutoAssist is mixing the ".length" property with the ".length()" method.

I would expect two separate entries in the auto-assist list. One for the .length property (with a 'P' icon), and one for the .length() method (with an 'M' icon).

_Please tell us what you were doing and what went wrong_

## Version info

Please paste the output of running `flutter doctor -v` here (available from the command
line or from `Tools > Flutter > Flutter Doctor`). It will provide the version of the
Flutter framework as well as of the IntelliJ plugin.

Microsoft Windows [Version 10.0.17134.228]
(c) 2018 Microsoft Corporation. All rights reserved.

C:\Users\rtola\AndroidStudioProjects\flutter_app>flutter doctor -v
[√] Flutter (Channel beta, v0.5.1, on Microsoft Windows [Version 10.0.17134.228], locale en-US)
• Flutter version 0.5.1 at C:\Users\rtola\dev\flutter
• Framework revision c7ea3ca377 (3 months ago), 2018-05-29 21:07:33 +0200
• Engine revision 1ed25ca7b7
• Dart version 2.0.0-dev.58.0.flutter-f981f09760

[√] Android toolchain - develop for Android devices (Android SDK 28.0.1)
• Android SDK at C:\Users\rtola\AppData\Local\Android\sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-28, build-tools 28.0.1
• Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b02)
• All Android licenses accepted.

[√] Android Studio (version 3.1)
• Android Studio at C:\Program Files\Android\Android Studio
• Flutter plugin version 28.0.1
• Dart plugin version 173.4700
• Java version OpenJDK Runtime Environment (build 1.8.0_152-release-1024-b02)

[√] Connected devices (1 available)
• Android SDK built for x86 • emulator-5554 • android-x86 • Android 9 (API 28) (emulator)

• No issues found!

C:\Users\rtola\AndroidStudioProjects\flutter_app>