com.intellij.tasks.ui.TaskDialogPanelProvider

It should be there (see https://github.com/JetBrains/intellij-community/blob/master/plugins/tasks/tasks-api/src/com/intellij/tasks/ui/TaskDialogPanelProvider.java)

Which IDE build do you use to compile against?

https://github.com/rieonke/idea-git-flow/blob/develop/build.gradle

Might be an issue with idea-git-flow plugin gradle script on case-sensitive systems. 
Plugin IDs in this case should be "git4idea" and "tasks".

IntelliJ IDEA 2018.2.3 (Ultimate Edition)
Build #IU-182.4323.46, built on September 3, 2018
JRE: 1.8.0_152-release-1248-b8 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
macOS 10.13.6

So what I did was I created a new plugin project.

Copied the source code into the sources directory and copied the plugin.xml file.

I added the git4idea.jar as a library.

I can't compile because IntelliJ complains that the classes are not available. 

import com.intellij.tasks.ui.TaskDialogPanel;
import com.intellij.tasks.ui.TaskDialogPanelProvider;

If I go to the platform-api.jar file, there is no ui package under tasks.

The plugin.xml file complains that the tasks.dialogPanelProvider entry below is not allowed here.

<extensions defaultExtensionNs="com.intellij">
    <applicationService serviceInterface="gitflow.Gitflow" serviceImplementation="gitflow.GitflowImpl"/>
    <applicationService serviceInterface="gitflow.GitflowState" serviceImplementation="gitflow.GitflowState"/>
    <projectConfigurable instance="gitflow.GitflowConfigurable" />
    <tasks.dialogPanelProvider implementation="gitflow.ui.GitflowTaskDialogPanelProvider" />
</extensions>

plugin.xml file:

<idea-plugin >
  <name>Git Flow Integration</name>
  <id>Gitflow</id>
  <description>Git Flow Integration</description>
  <version>0.6.5.2</version>
  <category>VCS Integration</category>
  <vendor url="http://www.opherv.com">Opher Vishnia</vendor>

  <depends>com.intellij.modules.vcs</depends>
  <depends>com.intellij.tasks</depends>
  <depends>Git4Idea</depends>

  <idea-version since-build="182.0" until-build="182.*"/>

  <actions>
    <action id="Gitflow.OpenGitflowPopup" class="gitflow.actions.OpenGitflowPopup" text="Gitflow Operations Popup..."/>

    <action id="Gitflow.InitRepo" class="gitflow.actions.InitRepoAction" text="Initialize Gitflow Repository"/>

    <action id="Gitflow.StartFeature" class="gitflow.actions.StartFeatureAction" text="Gitflow: Start Feature..."/>
    <action id="Gitflow.FinishFeature" class="gitflow.actions.FinishFeatureAction" text="Gitflow: Finish Feature..."/>
    <action id="Gitflow.PublishFeature" class="gitflow.actions.PublishFeatureAction" text="Gitflow: Publish Feature"/>
    <action id="Gitflow.TrackFeature" class="gitflow.actions.TrackFeatureAction" text="Gitflow: Track Feature..."/>

    <action id="Gitflow.StartRelease" class="gitflow.actions.StartReleaseAction" text="Gitflow: Start Release..."/>
    <action id="Gitflow.FinishRelease" class="gitflow.actions.FinishReleaseAction" text="Gitflow: Finish Release"/>
    <action id="Gitflow.PublishRelease" class="gitflow.actions.PublishReleaseAction" text="Gitflow: Publish Release"/>
    <action id="Gitflow.TrackRelease" class="gitflow.actions.TrackReleaseAction" text="Gitflow: Track Release..."/>

    <action id="Gitflow.StartHotfix" class="gitflow.actions.StartHotfixAction" text="Gitflow: Start Hotfix..."/>
    <action id="Gitflow.FinishHotfix" class="gitflow.actions.FinishHotfixAction" text="Gitflow: Finish Hotfix"/>
    <action id="Gitflow.PublishHotfix" class="gitflow.actions.PublishHotfixAction" text="Gitflow: Publish Hotfix"/>
  </actions>

  <project-components>
    <component>
      <implementation-class>gitflow.GitflowComponent</implementation-class>
    </component>
  </project-components>

  <extensions defaultExtensionNs="com.intellij">
    <applicationService serviceInterface="gitflow.Gitflow" serviceImplementation="gitflow.GitflowImpl"/>
    <applicationService serviceInterface="gitflow.GitflowState" serviceImplementation="gitflow.GitflowState"/>
    <projectConfigurable instance="gitflow.GitflowConfigurable" />
    <tasks.dialogPanelProvider implementation="gitflow.ui.GitflowTaskDialogPanelProvider" />
  </extensions>
</idea-plugin>

>I added the git4idea.jar as a library.
But you didn't add tasks.jar which is a separate plugin as well.

Thanks so much, that did it.