No milestone now: This will not block Dart 2.

The clarification may still be needed, and will be considered later.

Interestingly, "concatenate" is used in a few places w.r.t Strings:

Adjacent strings are implicitly concatenated to form a single string literal.

Dart also supports the operator + for string concatenation.

However, the use of the concatenation operation is still discouraged for efficiency reasons. Instead, the recommended Dart idiom is to use string interpolation.

It is possible to embed expressions within non-raw string literals, such that these expressions are evaluated, and the resulting objects are converted into strings and concatenated with the enclosing string. This process is known as \Index{string interpolation}.

What's interesting here is that efficiency comment. The "concatenation" found in string interpolation must be different from the "concatenation operation," if it is more efficient.