Describe the bug
Editor stylesheet CSS specificity issues.

To Reproduce

1. Add editor stylesheet as per the documentation.
2. Add a basic heading tag eg h2
3. An example of the generated style I'm using which works fine:

.editor-block-list__block h2 {
   font-size: 2rem;
   line-height: 1.16666667;
}

4. The line-height rule is being overruled by a few rules in core with higher specificity:

.wp-block-heading h1.editor-rich-text__tinymce, .wp-block-heading h2.editor-rich-text__tinymce, .wp-block-heading h3.editor-rich-text__tinymce {
   line-height: 1.4;
}

.editor-rich-text__tinymce.mce-content-body {
    line-height: 1.8;
}

Expected behavior
Core styles should have less specificity to allow for an easier time adding a comprehensive custom editor stylesheet.

Could the above rules not be written like:

.wp-block-heading h1, .wp-block-heading h2, .wp-block-heading h3 {
   line-height: 1.4;
}

.editor-rich-text__tinymce {
    line-height: 1.8;
}

This allows the editor stylesheet to correctly trump the core styles.
This is only a brief example, but I'm sure there are others. I'd be happy to list all problematic styles that I find if it helps.

Additional context
Gutenberg 3.8 and WordPress 4.9.8