This reverts commit 8dbb84f4.

Issue #3127

Issue #3991

The Error after the while(true) loop is not reachable and should
be removed. The "this.iterator" property should be given an
initial value of 0. This fixes two warnings reported by Closure
Compiler.

cm.save() might not exist if CodeMirror is not created from a
textarea. This adds an additional check to prevent TypeErrors.

Closes #3806

Closes #3803

zt was placing 40% of the line above the visible area, and zb was placing it too high.

Closes #3689

Closes #3684

According to http://usejsdoc.org/about-getting-started.html, jsdoc comments must start with /**

Fixes #3449

- Use 'unshift' instead of 'push' to ensure newly mapped commands come first (and hence take precedence over previously mapped commands)

https://github.com/codemirror/CodeMirror/issues/3268

https://github.com/codemirror/CodeMirror/issues/3237

https://github.com/codemirror/CodeMirror/issues/3219