All flavors must now belong to a named flavor dimension

FAQ:

All flavors must now belong to a named flavor dimension. Learn more at https://d.android.com/r/tools/flavorDimensions-missing-error-message.html

解决：

看官方文档：

Plugin 3.0.0 includes a new dependency mechanism that automatically matches variants when consuming a library. This means an app's debug variant automatically consumes a library's debug variant, and so on. It also works when using flavors—an app's redDebug variant will consume a library's redDebug variant. To make this work, the plugin now requires that all flavors belong to a named flavor dimension —even if you intend to use only a single dimension. Otherwise, you will get the following build error:

2.Flavor Dimensions变更

Android Plugin3.0的依赖机制：在使用library时会自动匹配variant（debug, release)，就是说app的debug会自动匹配library的debug，相信大多数人也像我一样，当library多了，不会手动选择每个Library的variant。现在好了，它会自动匹配了。同样如果使用flavor的时候，比如app的redDebug同样会自动匹配library的readDebug。虽然有这样的优势，但是在使用flavor时，必须定义flavor dimension,否则会提示错误：

Error:All flavors must now belong to a named flavor dimension.
The flavor 'flavor_name' is not assigned to a flavor dimension.

• 这个错误，我也已经在使用Android Studio3.0以后碰到好多次了。现在使用flavor，必须像下面一样配置：

// Specifies a flavor dimension.
flavorDimensions "color"

productFlavors {
     red {
      // Assigns this product flavor to the 'color' flavor dimension.
      // This step is optional if you are using only one dimension.
      dimension "color"
      ...
    }

    blue {
      dimension "color"
      ...
    }
}

• 注意：如果library有两个dimensions:color,shape,但是app只有color,那么会如下的编译错误：

Error:Could not resolve all dependencies for configuration ':bar:redDebugCompileClasspath'.
Cannot choose between the following configurations on project :foo:
  - redCircleDebugApiElements
  - redSquareDebugApiElements
  ...

• 在APP使用flavorSelection选定使用某个flavor dimension，注意如下配置：

android {
  ...
  // The flavorSelection property uses the following format:
  // flavorSelection 'dimension_name', 'flavor_name'

  // Chooses the 'square' flavor from libraries that specify a 'shape'
  // dimension.
  flavorSelection 'shape', 'square'
}

ref docs:
迁移到Android Studio 3.0
http://blog.csdn.net/ncuboy045wsq/article/details/73521856
https://developer.android.com/studio/build/gradle-plugin-3-0-0-migration.html